Morphling says:
On my part it doesn't seem to have registered that I've joined on the Games drop-down.
not sure if that is your case, but the Games drop-down at the top of the GP page shows games that you have manually bookmarked (by selecting the bookmark icon top-right of a game title bar, in that game's Game Details page).
The drop-down games list can also only show ten bookmarked games, I believe. So, some games might not show there because of that.