It is possible sometimes that the guide can manage for some people to come down with one of the guides if they are tired or unable to continue. However if there is an accident or someone becomes incapacitated then he will require that everyone in the team assists with the descent, and he will likely call a snow machine to come up as far as possible to help evacuate the casualty.
The ideal aim is to keep everyone together. The guide will make his or her decisions based on safety, and cannot allow 'fast' and 'slow' teams just for the sake of it. Neither should the guide succumb to any calls from clients for the group to split up on summit day, however close to the summit, if he deems that it will affect the safety of any individual in the team.
If some people want to go down because they are too tired or exhausted and it is possible to split the team safely, then one guide will descend with those people and the rest can continue to the summit with the other guides. However the aim is to maintain a safe ration of guide to clients, and not end up with too few guides on the summit ascent. So the number of groups can descend is limited. If it reaches a point that the ratios become unworkable and safety is threatened then everybody will have to descend. This is why generally the guide will mention certain points on the mountain where a turnaround is possible, for example in the Saddle.