Bring good lights for every rider if you manage to go longer than daylight. And enough power to keep the lights running for several hours.
If you can do 50% and still walk afterwards, then you likely can do the whole ride.
The difficulty of the ride of course also depends on the terrain.
I don't know what your group dynamics are, but I'd hope that you don't drop anybody unless you choose a town or something as a convenient bail out point.