Can you re-check what you doing and post the request with the results you are getting.
According to the code,
The following example adds a user, adds two shared groups (if not existing) and adds the user to both groups.
http://example.com:9090/plugins/userService/userservice?type=add&secret=bigsecret&username=kafka&password=drowssap&name=franz&email=franz@kafka.com&groups=support,financebigsecret&username=kafka&password=drowssap&name=franz&email=franz@kafka.com&groups=support,finance
The following example adds new roster item with subscription 'both' for user 'kafka' and adds kafka to roster groups 'family' and 'friends'
http://example.com:9090/plugins/userService/userservice?type=add_roster&secret=bigsecret&username=kafka&item_jid=franz@example.com&name=franz&subscription=3&groups=family,friendsce?type=add_roster&secret=bigsecret&username=kafka&item_jid=franz@example.com&name=franz&subscription=3&groups=family,friends
Note the difference between a roster group and a shared group