There is nothing wrong with the strong leaving Muzdalifah with the weak if they are a single group
Question

                Many groups leave Muzdalifah at the end of the night and before dawn because they have women and elderly people with them. Is it permissible for the strong to leave with them, or should they stay there until they have prayed Fajr?.
Praise be to Allah.
    If the people are travelling in the same vehicle then they 
    come under the same ruling. If they leave at the end of the night because of 
    the weak and the women, then they may leave all together, because separating 
    them will cause hardship for them, but Islam is a religion of ease. If this 
    bus has sixty passengers, for example, and twenty of them are weak people 
    who need to leave sooner so that they can stone the jamarah before dawn 
    breaks, then it is permissible for the rest to do likewise, so these forty 
    people may go with them on the bus because they are all one group, and separating them would cause them hardship. End quote.