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:
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Add users in user mode.
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Ask a Confluence administrator to add the users in group mode.
Bug report at Atlassian: