I have four ability groups for maths and literacy, not the same groups for each. Children know which group they are in, but the groups are also on the wall so they can check. I work with one group, TA works with another. We kind of evolved to have our own tables, just happened that way so when working with us the children sit with us. No specification of which chair. If not with us children have been sitting in whichever area I directed them to, in their groups. From next week they will be choosing from a selection of activities so the two groups working independently may well mix up. For all other lessons children generally work in random groups and choose where to sit.