refactor: add user to group instead of group to user

no functional change, just improving readability

benefits/core/admin.py

@@ -151,12 +151,12 @@ def add_google_sso_userinfo(user, request):
 def add_staff_user_to_group(user, request):
     if user.email in settings.GOOGLE_SSO_STAFF_LIST:
         staff_group = Group.objects.get(name=settings.STAFF_GROUP_NAME)
-        user.groups.add(staff_group)
+        staff_group.user_set.add(user)
 
 
 def add_transit_agency_staff_user_to_group(user, request):
     user_sso_domain = user.email.split("@")[1]
     if user_sso_domain:
         agency = models.TransitAgency.objects.filter(sso_domain=user_sso_domain).first()
         if agency is not None and agency.staff_group:
-            user.groups.add(agency.staff_group)
+            agency.staff_group.user_set.add(user)