It is easy to build up a friends list. I have only played xbox live
for a total of maybe 20 hours or so, and I am in a clan and have a
friends list with like 25 to 30 people on it. Just play in ranked
games or training match games, then at the end when you have the
post-game carnage report, send friend requests to anyone you thought
was a good player. Most of the friend requests I have sent have been
accepted. Once they are on your friends list, you can join any game
they are in even if they don't invite you. (unless the game is set to
invitation only or something). Then it builds on itself. As you start
playing in custom games with those people, you will meet more people
who like to play in custom games, and you can send them friend requests
as well. Anyone you have played against recently will be in your
recent players list which is in the same place as your friends list.
You can send friend requests from there as well.
You can send me a friend request if you want. Gamertag is poolhustler.