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Go to the groups tab to do CRUD on groups and to add/remove users. The group 'Administrators' is reserved for Synapse administrators. Any Synapse user added here will have full administrative privileges in SyanpseSynapse.
There is generally no need to manage Users via Crowd, as they can be created and updated in via Synapse or the REST API. Users may be removed, if necessary, via the Crowd console however.