When you have allocated sufficient time to get to know each other, through covering lots of planning, preparation, support, training, then everyone knows whether they fit in or not.  People organically recognise what themselves and others can bring to the group, and people recognise whether they agree or not with the core principles of how they will work together, and whether they are comfortable and well supported.  Some volunteers may recognise, that after a period of time, they are not able to fully commit to the service and decide to withdraw. That’s ok.