Add StudyGroups with ExternalUsers only

db/migrate/20181122090243_create_study_group_memberships.rb

@@ -0,0 +1,8 @@
+class CreateStudyGroupMemberships < ActiveRecord::Migration[5.2]
+  def change
+    create_table :study_group_memberships do |t|
+      t.belongs_to :study_group
+      t.belongs_to :user, polymorphic: true
+    end
+  end
+end