Fix group members import functionality

When managing a group, there is a button that allows to add import members from another group.

currently, the functionality seems bugged:

• list displays 'undefined()' instead of group name
• only groups that user is member of are listed (is it intended ? admins will not have access to all groups)
• when members are imported, there is no call to 'users.setMemberOf' method, so plugins (keycloak, rocketChat) will not take these new permissions into account.