Add Project#members_among to obtain only authorized users of the project

app/models/project.rb

@@ -1365,6 +1365,18 @@ class Project < ActiveRecord::Base
     end
   end
 
+  # Filters `users` to return only authorized users of the project
+  def members_among(users)
+    if users.is_a?(ActiveRecord::Relation) && !users.loaded?
+      authorized_users.merge(users)
+    else
+      return [] if users.empty?
+
+      user_ids = authorized_users.where(users: { id: users.map(&:id) }).pluck(:id)
+      users.select { |user| user_ids.include?(user.id) }
+    end
+  end
+
   def default_branch
     @default_branch ||= repository.root_ref if repository.exists?
   end

spec/models/project_spec.rb

@@ -3995,6 +3995,49 @@ describe Project do
     end
   end
 
+  context '#members_among' do
+    let(:users) { create_list(:user, 3) }
+    set(:group) { create(:group) }
+    set(:project) { create(:project, namespace: group) }
+
+    before do
+      project.add_guest(users.first)
+      project.group.add_maintainer(users.last)
+    end
+
+    context 'when users is an Array' do
+      it 'returns project members among the users' do
+        expect(project.members_among(users)).to eq([users.first, users.last])
+      end
+
+      it 'maintains input order' do
+        expect(project.members_among(users.reverse)).to eq([users.last, users.first])
+      end
+
+      it 'returns empty array if users is empty' do
+        result = project.members_among([])
+
+        expect(result).to be_empty
+      end
+    end
+
+    context 'when users is a relation' do
+      it 'returns project members among the users' do
+        result = project.members_among(User.where(id: users.map(&:id)))
+
+        expect(result).to be_a(ActiveRecord::Relation)
+        expect(result).to eq([users.first, users.last])
+      end
+
+      it 'returns empty relation if users is empty' do
+        result = project.members_among(User.none)
+
+        expect(result).to be_a(ActiveRecord::Relation)
+        expect(result).to be_empty
+      end
+    end
+  end
+
   def rugged_config
     Gitlab::GitalyClient::StorageSettings.allow_disk_access do
       project.repository.rugged.config