I play a chanter, as well as other melee toons... I've never been charmed on my chanter (MR is around 150-200), however as a Paladin, I've been charmed and like most others to reply, it was in the Gorge... Nasty...
Now a couple things... When the charmed person types (e.g. /g HELP, I'm stuck, etc...) the response is NOT what they typed... Look for a group tell that says something along the lines of "I serve my master <insert name of mob>"...
As with the methods posted above on how to deal with this, I'd say that the first problem here is the group you were in wasn't paying attention to what they were doing. I don't know about you, but when I'm getting attacked and I'm not the MA, I usually can see it happening and noticing it's Player <insert name> tells me he's charmed...
So here's where I see a few things that could/should have happened:
1. The chanter is probably the busiest class in the game... If they're not busy and swapping out spells every few mins for other spells and keeping an eye on what's going on, then they're not that great of a chanter. The chanter should have noticed that Player X was attacking Player Y and should have either mez'd or rooted... From my experience with this, a stun is not that useful and I'd much rather mez than stun (take into effect cast times, resists, etc.) But that's my thought.
2. The cleric should have noticed Player X was beating on Player Y and a stun should have been in order. Expecting the enchanter to do it all and deal with mobs is a huge mistake. The same goes for other classes as well. If you have a spell that can help, you should always have it loaded.
3. Was there a druid? What about a root and a tell to the group to back away from charmed player? When I was on my paladin in the gorge, I was destroying my group members. The druid there just rooted me, told the party to back away sit back and wait for the charm to break once the main mob was dead.
4. Honestly (and I'm probably going to get flak for saying it), I'd dispell the sucker... Lets be honest here... If you're not in a Hard LDON, then your group can do just fine without having 1 out of 6 players buffed up... Besides, it might stall your group all of about 5 minutes to rebuff the tank... Depending on casting time, I'm might opt for this option before all others and rebuff the tank as we move on...