In short, I think most clubs don't need to do anything. A lot of people strongly prefer to ride outdoors and they enjoy the social aspects. You can work one and sometimes two structured training sessions into a typical group ride schedule. Furthermore, I don't think a lot of people will be riding indoors only through the year. Thus, clubs won't lose many (or any) people to indoor training. More discussion is below. Optionally, clubs might organize race practices as a substitute for structured training, although this takes effort.
Working structured sessions into a club schedule
On their YouTube channel, Road Cycling Academy discussed how individual riders can work structured training into your outdoor season. They recommended that riders who want to do this cut down on group rides, but not eliminate them. For example, if you did 3 group rides per week, you might cut down to 2 and add one structured training session.
I'm confident that a lot of riders will still see a significant benefit with only one or two structured sessions per week during the warmer season. Therefore, structured training is very unlikely to supplant outdoor riding entirely, or even to a great extent. Now, there will be some athletes who actually need multiple structured sessions to progress - they're likely to be a minority, and they'll be pretty advanced athletes.
Many clubs have one or two fast rides in the middle of the week, and long rides on one or both weekend days. In my experience mixing structured sessions and club rides:
- Two structured training sessions during the week, no harder weekday rides, and long rides on the weekend are doable. Easy weekday rides would be fine.
- One structured session, one harder group ride in the middle of the week, and weekend group rides are also doable. Sometimes, my recovery suffers if I try a structured workout too soon after a long ride. If your club rides Wednesday and weekends, then scheduling can be awkward. I've done Tuesday morning structured plus hard ride Wednesday night.
- Two structured training workouts, plus one or more harder weekday and weekend rides may be possible, but they stress the body considerably. Recovery won't be complete. Younger folks might get away with this, but I feel like it might not be optimal even for them.
- Generally, you aren't likely to be able to do a productive structured workout on the next day after a long group ride. Group rides with a lot of town line sprints are very fatiguing, and they require some recovery. Few people can do group riding on Saturday and structured training on Sunday. Some discussion is below. The reverse, e.g. structured session Saturday, group ride Sunday, might be OK if you're willing - we are in the draft a lot, and you might opt not to contest sprints as aggressively. I've done VO2max sessions the day before moderately demanding group rides, and it can be OK. A long threshold session would probably be too much.
Teams that want to organize rides where competitive cyclists can get some targeted training might think about doing hill repeats (like VO2max work), or hosting a regular time trial (like threshold work). A practice criterium will let people get a lot of anaerobic work in, even if they're in the draft.
Accommodating Zwift natives with fewer outdoor cycling skills
Amateur clubs may encounter cyclists who started their road cycling careers on Zwift, and who may have less or maybe no experience on the road. There are a few of them out there. I met one such rider last Saturday, albeit he had MTB experience.
Pro road teams deliberately try to recruit from these channels, but indoor athletes will have a learning curve in the pro peloton, and they may struggle. There are a lot of soft skills, e.g. reading the race, pack positioning, basic procedures of how to ride in a very large and very fast group, handling an actual bike.
While Zwift and other platforms obviously do simulate a draft, it will always be inherently a bit different from outdoors. There are transmission delays between your computer and Zwift's server (latency), so all control inputs are a bit delayed on Zwift. Zwift doesn't increase your trainer resistance when you're in clear air, whereas you will most certainly feel it in real life. On Zwift, most people lack steering. Holding a wheel in Zwift feels markedly different from real life as a result of all of this. So, your Zwift instincts won't directly translate to outdoor pack riding, and vice versa.
In the old days, the norm may have been that you had to earn your turns, i.e. prove yourself before they deigned to let you take a pull at the front. This attitude may not sit well with people today, even if they started as Zwift-only athletes. It is objectively a bit exclusionary.
If someone is coming from Zwift to an A group ride (e.g. a lot of people whose power levels are similar to US Cat 3s or faster), then that attitude is probably justified. Your ability to handle your bike well will degrade at very high effort levels - I'm an experienced cyclist, and yet I've noticed this when I try to hang with the A rides in my area. Also, they will probably be less confident holding a wheel closely, and thus they will get less of a draft. This means burning more energy and tiring sooner.
Otherwise, lower level rides can probably accommodate Zwift natives. Fellow riders should be gentle, but definitely look out for them and instruct them on bike handling and group etiquette. Understand that they are less able to point out obstacles, because they may be focused on controlling their bike - this is OK because there is redundancy in the group, so we don't need every single person to pass a warning down. Understand that they will probably not follow wheels as closely as they should - encourage them to close gaps without being too critical. If you are at the front with one of them, communicate your thought process to them, let them know when you're pulling off, remind them that they can pull off. Clubs can consider asking these riders to join easier rides to build outdoor skills as well - but do be aware that easier rides may not be sufficient training for faster rides. The physical strain is less, and the group dynamics may be less complex.
Misc: group rides and training
Group rides don't typically offer enough training stimulus in the specific power zones that we target in training, yet they can be quite fatiguing at the same time. Consider the zone distribution from one of my group rides. It was about 60 miles and 3+ hours, with a pretty fast group (the faster riders were about equivalent to US cat 3s). I was taking pulls at the front, but not a lot of them.
Someone of my experience might try to target at least 40-45 minutes at threshold OR at least 16-20 minutes at VO2max in a single session. In this ride, I spent only 12 minutes at threshold and 8 minutes in the VO2max zone. With that zone distribution, I would hit a plateau if I did only group rides. Most of the ride was at recovery pace thanks to the draft. Yet I also spent 8 minutes at anaerobic power, which is pretty fatiguing.
If you want, say, more threshold time, you could go to the front and pull all day. That's probably within norms for some clubs or some groups in the club (e.g. you could help a slower group go faster). But you can't count on that norm, you can't count on your threshold pace being right for the rest of the group, and others may want to pull.
Thus, your standard group ride is usually a bad substitute for structured training. Most riders will just build their training plan while assuming they'll do some group rides separately. I think that some pro roadies may do an endurance ride in a group, then break up for their own individualized intervals, then regroup and head home together. So, a club that's got a lot of racers might think about this arrangement. More likely, hosting a regular practice TT, crit, or hill session as described above should be enough for most clubs. I do feel like you can get a fair bit of anaerobic threshold or sprint work on group rides. The amount of stimulus has been enough for me to ignore this in my structured training. I suspect many people might feel similarly.
Indoor training and talent discovery
While not originally asked, there are some things to say about the effects of indoor training on the cycling population. In the US, most people probably are exposed to cycling as a collegiate sport. But not everyone gets into cycling that way. On the pro side, indoor training has let teams discover some riders who didn't come in through the usual development channels, i.e. they have more talent, which can lead to closer competition between teams, which is generally good for fans.
Talent development has also been affected. I think the consensus is that structured training (outdoors or indoors) has helped the entire peloton get fitter, and also maintain more fitness over the winter. In some ways, that's a benefit to the athletes and the fans. There are also trade offs: many riders are able to fiercely contest the early season races, whereas in the past they would use them as training races. Otherwise I think it's a clear net positive.