Correct, I don't have any roles associated with that group. When I try to add the user specifiying the Users group, I get the same error:
useradmin user add my_user -g Users
Could not add user <my_user>. Error: Invalid parameter
But it shows that the Users group has the audit Role:
useradmin group list Users
Info: Ordinary Users
Allowed Capabilities: api-snmp-get,api-snmp-get-next
What's gone wrong here?