Roster syncing will sync or mirror changes in any roster across all the linked groups. If James is added to one group, he is added to all synced 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 Syncing click the blue Edit button in one of the groups and choose Roster Syncing.
On the next page, choose the group(s) with which you want to sync rosters.
Yep, it is that easy.