Create a user list to quickly add a specific group of people to a thread or forum. Anyone in your organization can create a user list, and anyone can view and join them.
To create a user list in your organization:
Go to your organization in the left-side navigation
Select Create user list at the top-right corner and follow the instructions
You can also view, join and leave any user lists in your organization by viewing the User Lists tab in the Members section of your organization's settings.