Since you say UserWS - sounds like you are using the old WSAPI?
There is a separate UserGroupWS endpoint for managing group membership. - eg https://<yourserver>/ustorewsapi/UserGroupWS.asmx?op=AddUserToGroup
I understand using WSAPI if modifying an older app, but if creating something new, I would recommend swapping to use the REST APIs. In this case, there are also two endpoints for either users and usergroups.