In the internet game RuneScape I have seen two people attacking the same creature at once but when I try to play with my friend it won't let us attack the same creature so how do you play together?

Answer 1

That only applies to some monsters for example if your fighting a "moss giant" in the varrock dungeon you and your friend can attack that same monster. But if you're in the karamja dungeon where the most common place to kill lesser demons only one person can attack a lesser demon at a time. Most players don't like it when you attack the monster they are already attacking so try to stick on your own. But if the player is being a jerk i attack their monster for fun.