Our playgroups are pretty small, and they tend to organise their own cleanup.
Definately do a roster, include everyone, and if someone misses a go, they swap with someone else. Just put the roster up, no questions.
Welcome to the world of kindy/playgroups. Its a bit sad, but there will always be a core 'action' group and a much larger group of people that don't participate (but usually the first to complain).

Pick away, playgroups are so very important to the community and I'm glad to help.