Adding users in group mode does not work for space admins in Confluence 8.5.x
Due to an issue with an internal API endpoint in Confluence 8.5.x, retrieving users from Confluence groups returns an error for non-Confluence-Administrators and affects the group mode function in Team Admin. Adding users in group mode is not possible for space admins without Confluence administrator permissions.
Workarounds:
Add users in user mode.
Ask a Confluence administrator to add the users in group mode.
Bug report at Atlassian:
https://jira.atlassian.com/browse/CONFSERVER-98582