This is perhaps the most annoying thing when playing as a healer. I have quit after the team wiped repeatedly and blamed me for it (because it's always the healer's fault when things go wrong). What was actually happening is what you described - the three other players (a guardian and two sentinels) leaped in and all attacked the boss. The adds and other beasties were ignored, and when they that happens they always attack the healer. Then the others screamed abuse at me because I was spending all my heals on myself.
I considered lecturing them (especially the tunnel-vision sentinels, one of whom kept using Guarded by the Force even when he was at full health), but then thought it wasn't worth it and quit. My one and only rage quit. Most of the time I'm happy to go on even in the face of repeated wipes (there is in fact no better feeling in the game than when you finally beat a foe that had been kicking your butt repeatedly), but being blamed for other people's mistakes was just too much.
I have been there, so many times lol.
Most recently, the group kept wiping because the DPS was dying faster than I could heal them. Since I'm in BH gear and have done all the HMs to death, that points to a problem with them, not my heals. They were not moving out of the giant circles on the ground and kept piling up on the boss instead of taking care of the adds. But of course, they started trying to "help" me heal them by giving advice. "Stop putting down your heal circle, it wastes mana." "Did you try keeping HOTs on us?" "Try shielding us" -- all of which I was already doing. Except for the heal circle part. Telling a corruption sorcerer not to use their heal circle is like telling them not to heal at all, lol.
All my attempts to tell them what to do were of course ignored so I finally said I needed to go, quit, and requeued. :P