You can only loot 1 boss per week per difficulty. So if you kill the first boss with one group, you can join another group and get loot from the second and so on. You cannot get loot from the same boss twice, but you can get loot from the same boss in Normal and again in Heroic.
The lockouts work just like flex and LFR did in MoP. You can kill bosses as many times as you want, but you can only loot them once per difficulty. Doesn't matter if it's in the same group.
What about bonusroll? U have killed kargath with a raid. U go the next day with another grp and kill kargath again. Can you use bonusroll on the 2nd kill?