Click the blue "new gaming session" button to post your next gaming session.
At work? Post it now so others can see it and join so you're ready to go later. You have to manually invite everyone using the XBL/PSN messaging system, so make sure to do that a little before your game time. As the session creator, the others are expecting you to invite everyone into a party.
Creating a gaming session will automatically email all members of your group when you post a gaming session, so don't double post or create fake gaming sessions.
If you don't want to get these emails, just click "My Account" -> Edit Profile, drop down "more notification options," and uncheck the box that says "[Email] New Group Game
NOTE: If you want more information on Game Session types (Default, Group Only, and Friends only), check out this help article on Game Session Types
For information on inviting users check out How to invite users