(Verb) Join up, become part of a group, generally a group that is going to go do something mischievous and/or unlawful. In an MMO, it means to become part of a questing/PvE party or PvP party.
Not that this is completely different from throwing in the towel.
Dan: Bob and I are gonna go somke some weed and egg a house, you coming?
Frank: Sure, I'll thrown in for that shit any day.
Lamerrr: LF2M BRD
Dissonance: BRD? I'll throw in, as long as you don't ninja loot anything.
To contribute money for a specific purpose or for the good of the group. Informal
The phrase may derive from the act of passing around a hat, into which contributors would "throw in" some money.
"Another six-pack of Hamm's? I'll throw in for that!"
Not: "There's orphans starving in Haiti? I'll throw in for that!" Situation too formal or grave.