Might work like some MMORPG where XP is awarded for the %of the damage done to the enemy, so If something gives 100xp, and you've done 65% damage on him and one of your teammate did the other 35%, you'll get about 65% of the xp, and your teammate will get 35%, Might get a little bit more for the finishing blow, and might also get some xp (c-bill) for the "Assist".
This prevent "abuse" of steal killing, but yet still reward teamwork.
You might also simply get C-bill simply for damaging an enemy, no need to get it killed, since there is no "healt/Armor regeneration" system in MWLL like you can find in most recent FPS (CoD, Red Faction, Gears of War etc).