There are reasons why you can't join or leave parties while engaged (and it'0s like that in many MMOs), as the possibility opens the way for exploits.
Actually I think it's done this way because cutting the arm off was easier than bothering to work around it. There are workarounds without inconveniencing players the way the current system does: you can simply look at games like WoW or even FFXI. What you do is you isolate the rooms or encounters where there should be no interference once the battle has started.

Not every encounter needs to be like this, so you don't completely have to remove the ability to invite people like they did here.

If you need some examples: In WoW's case, the boss's door to his room usually closes, or something along those lines. In XI's case, the event either transports you to an isolated event area where new party members can't enter, or puts a debuff on the party at the time of engaging in order to let you attack the enemy (the Confrontation system).

I implementing these things takes time, however, which seems obvious they didn't have. I'd be surprised if you still can't invite people mid-battle by the time the year ends (not counting those specific times where they actually don't want you to).