How do you invite people to a game on xbox live that ARENT your friend?

Sometimes when i'm playing a game on xbox live some people that i DONT know will invite me to play with them. i was wodering how you do that so i can invite people that ARENT my friend, or just see who is playing the same game as me when i'm playing it.
How do you invite people to a game on xbox live that ARENT your friend?
Open your guide button, select your friends list and move to the right to see your recent players list. These are all of the non-friends you have recently played with. You can then open their gamer profile and invite them to a game if they allow it. You may not want to do this since most people get annoyed when people they don't know send them invites.