I find it extremley annoying when people just run into my battle without asking, attract another enemy, and flee. I understand the idea of helping out, but if I'm low on health, the last thing I need is another enemy. Maybe you could add an option that prevents others from joining your batlle or only friends can join it. Or maybe even an option where you first have to send a request to join the battle and then the battler in the first space can either accept or decline.
:x 
No one likes it but you can't get rid of it.