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

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.