Add StudyGroups with ExternalUsers only

app/models/study_group.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class StudyGroup < ApplicationRecord
+  has_many :study_group_memberships
+  # Use `ExternalUser` as `source_type` for now.
+  # Using `User` will lead ActiveRecord to access the inexistent table `users`.
+  # Issue created: https://github.com/rails/rails/issues/34531
+  has_many :users, through: :study_group_memberships, source_type: 'ExternalUser'
+  has_many :submissions
+  belongs_to :consumer
+end