Roster synching will synch or mirror changes in any roster across all the linked groups. If James is added to one group, he is added to all synched groups. If Noelle is transferred from 2 year olds to 3 year olds, she is removed from all synched 2 year old groups and added to all synched 3 year old groups.
To set up Roster Synching click the blue Edit button in one of the groups and choose Roster Synching.
On the next page choose the group(s) with which you want to synch rosters.
Yep, it is that easy.