Personally, I used to find it intensely irritating when duoing with Lenah and in the middle of an evenly matched combat half a dozen rangers/shades would suddenly pop up and get involved.. but I also admit that its reassuring to know there's backup around just in case its needed

As has been stated already RvR is WAR, if I encounter a battle involving my realm-mates I will stop to assess the situation.. if my stalwart comrades are winning I will leave them to it, if they are in need of assistance I will wade in.
Everyone has their own opinion on when it is ok to add on someone elses combat, the issue is really how you deal with people adding.. a polite request along the lines of "Thanks for the help, but we prefer to fight alone if thats ok so please try not to join in next time" is going to be far better recieved than "FOAD leeching scum!"
It's very frustrating for our more casual RvR players when they try to help their realm-mates and are met with abuse rather than gratitude. I know a great number of people who have been put off the RvR aspect of the game purely because of this attitude.
Finally, a word of warning for those players who choose to throw abuse rather than ask politely..
Some time ago I was experimenting with my void eld in RvR, I saw an Alb and started casting, unaware he was being stalked by a pair of Rangers. Despite it being unclear who had targeted the Alb first I recieved a torrent of abuse and obscenities for "adding"....
...Unfortunately for the Rangers concerned, it had been my BB they had asked for Buff's at Ligen and, not wanting to compound the situation with further assistance, I removed their buffs... just as the albs friends arrived.
