You can add anyone to a playlist if they are already in your MultiTracks.com Account.
While on a Playlist, click Add People
Click the checkbox next to a person's name to add them to the playlist
To add everyone in your account to the playlist, click 'Add All People'
If you don't see someone listed in this list, and you'd like to add them, click 'Add your team to your account' to add them to your account. After you add them, you'll be redirected back to this pop-up to add them to this playlist.
For more information on adding people to your account, click here.