When trying to add a user to certain groups, I get the following error:
Unable to add the user to the group. They may not have the privileges necessary to join this group, or may already be a member in the maximum number of groups (512).
This user is a Creator (Level 2) and is part of my organization account. It seems like it's only certain groups. I'm the group owner, and the settings between a group I can add the user to, and a group I can't add them to, seem the same.
I have the same problem!
I had this same problem.
The issue is one of permissions.
Most likely the groups where users don't show up were probably created with as Shared Update groups.
But even Creators w/ Publisher lever access still seem to need more permissions. I had previously created an AnlaystManager user type that had many elevated permissions and once I set folks to that level, I could put them in the group. However, that was more of a test to understand the problem.
I ended up recreating the group without Shared Update, then I could turn it over to one guy with elevated permissions and he could then add folks into it.
Shared Update is a confusing thing...