Everyone has a maximum "enjoyable" power (in watts) that they can output for an certain time, more time means less power of course. A cycling champion might do 400W for 1 hour, a newbie perhaps 80W...
Problem is the newbie doesn't know how to listen to his untrained body, so he will cross the red line, especially if well motivated, and will get out of resources (glycogen) or get cramps, acid overdose, or get out of breath, etc.
So you need to micromanage the newbies the same way do with your own body when cycling, except you have much less feedback. So you need to chat with them, and very often tell them to go slower.
You need to know what is possible for your friends. No matter the motivation, you can not ask the impossible.
- know the power to weight ratio of the other guys
- know the gearing they have on their bikes
- thus, calculate the maximum slope.
For example, a newbie (1 hour P=100 watts) fat (m=120 kg including bike) :
max vertical speed = P/mg = 0.085 m/s = 5 m/min = 300 m/h
Let's suppose he has a granny gear of 32T front, 28T rear (a very bad choice for him), wheel circumference 2m, cadence 60 rpm :
minimum speed = (60 rpm)/60 * 2m * 32/28 = 2.28 m/s = 8.2 km/h
We will not make him stand on the pedals (too tiring) or use a lower cadence (not enough more muscle).
So, what maximum slope can this guy climb happily ?
300m every 8.2km = 3.6%
Yes I know, this isn't even a hill, for a seasoned cyclist it's pretty flat, but for a fat newbie on a bike with a transmission unsuitable for him, it's the maximum he can do. For a minute or two he can do 2x more, but not longer. Choose your routes appropriately.
If he has a better transmission (shorter gears) he can climb more percentage while staying at the same power, this means slower.
There is a very useful tool : the GPS' variometer which tells me my climb rate in m/min.
When I'm with MTB buddies who are faster climbers that me, I look at the variometer : I know how long I can last at various climbing speeds, so I know when not to follow them and go at my pace. They'll wait for me at the top, but not long, since I won't be out of breath.
When I'm with buddies who are slower climbers that me, I also look at the variometer : I know I shouldn't push them past "x" m/min, and follow that rule. It does work well !