On the update page, dont add the usergroups.UserGroup column to the WADAUsers recordset, but you do need to add the users.UserGroupID column.
create a new recordset that queries the usergroups table to return the UserGroup and UserGroupID columns.
Use that recordset to populate the user group select list, same as you did on the insert page.
In the Dynamic Select list options, in the Select Value Equal to section, click the lightning bolt icon and select the WADAUsers.UserGroupID recordset column.
In the update record behavior, bind the UserGroupID column to the user group select list.