Xbox Live games have two types of online matches: Ranked and Player. Ranked matches affect your scores on global leaderboards, as well as your TrueSkill
rank. Because of this, you can't invite friends in case you use it to boost your rank unfairly (there are exceptions, which I'll explain in a sec). Player matches don't have any affect on any leaderboards and you can invite anyone to join the game at any time.
The exception to inviting people to Ranked games is when the game uses a party system, which as you know from GTA4 (and maybe COD4 and Halo 3), lets you play as a team against other players/parties. You can only do team versus game modes though, so in GTA4 you're limited to Ranked Team Deathmatch, Cops n Crooks, Team Mafiya Work, Team Car Jack City and Turf War, but obviously if you play Player matches you can do any mode (which I usually do since most people in Ranked games are wankers anyway).