When looking for open games i like to join the same games that my friends are participating in but sometimes the same open games are not available even though we are looking at the same time. Why is this?

could be several reasons:

1) the game filled up and got started faster than you got there and is no longer listed.
2) the game has requisites which some of you don't fulfill.
3) you are blacklisted by the person who created the game.

About number 2, you can read more about it at the wiki:
http://wiki.warlight.net/index.php/Open_Seat_Prerequisites

ps is correct, but I just want to add on the prerequisites. If you open the game, click Settings, you can see what the game's prerequisites are. With this, you can tell exactly why that game isn't showing up for them.