343 (individual name) [MARN] in nearly every game. WYF?!
Those are bots that replace people who quit or who got dc’d untill Join in Progress will place a new player in your match. The [MARN] after the name tells you how good they are. MARN = Marine quality and ODST = ODST-quality (1 step better then marine), those two are the only ones that will fill in so far.