Add user to existing User List
1) Click to enter the "User Lists" view from the left-hand side bar.
2) The User List page will default to showing only the User Lists that have been created by you. To also view User Lists created by other people in your organization, click on the "Showing:" drop down menu and select "Everyone's Lists".
3) Find the User List.
4) Click the "Add User" button in the top right corner.
5) Select the pre-existing user(s) that you wish to add and click "Add People".
Note: If a user does not exist yet, you will first need to create them or have an Admin or user with Manage User permissions in your organization create them in order to add them to a User List.