Add StudyGroups with ExternalUsers only

app/models/study_group_membership.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class StudyGroupMembership < ApplicationRecord
+  belongs_to :user, polymorphic: true
+  belongs_to :study_group
+
+  validates_uniqueness_of :user_id, :scope => [:user_type, :study_group_id]
+end