Hi, the IDs in the users.list array are user IDs, while the IDs in the "groups" array are group IDs. More specifically, these arrays contain KinveyReference objects (http://devcenter.kinvey.com/rest/guides/datastore#RelationalData), which means that the following:
In REST API - can you document on here http://devcenter.kinvey.com/rest/guides/users#usergroups what list and groups mean, and what the IDs that you are adding there mean.
I think I have worked it out but its not explicitly documented
While there, the update groups doc currently says
This looks like something that has come out of how you auto-generate your help docs