-G grouplist  Set additional group memberships for an account.  grouplist
                   is a comma, space or tab-separated list of group names or
                   group numbers.  The user's name is added to the group lists
                   in /etc/group, and removed from any groups not specified in
                   grouplist.  Note: a user should not be added to their pri-
                   mary group with grouplist.  Also, group membership changes
                   do not take effect for current user login sessions, requir-
                   ing the user to reconnect to be affected by the changes.

srv ~ # groups u8852
u8852
srv ~ # pw usermod u8852 -G 'www'
srv ~ # groups u8852
u8852 www
srv ~ # pw usermod u8852 -G ''
srv ~ # groups u8852
u8852 www
srv ~ # pw usermod u8852 -G 'vk'
srv ~ # groups u8852
u8852 www vk
srv ~ #