I'll go against the grain and pretty much disagree with all the advice given in this thread.
You asked her to go KAYAKING. On a FIRST "date." In fact, that doesn't even sound like a date...sounds like a fun time, but doesn't sound intimate at all. Which is PROBABLY why she assumed it would be alright to invite people along.
YES, it was a RUDE move on her part to invite people along without consulting you first, but as far as the seduction part of this is concerned, I don't think it's a huge red flag.
I mean, if somebody invited me to go skydiving with them, and I knew another person who had been dying to go skydiving, I'd think to invite them along.
IMO a FIRST date should be something FAR more casual, FAR more intimate, and FAR LESS time consuming.
So you can go kayaking if you want, or you can cancel. IF you think you have enough game to game her somewhat in a group setting, go for it. Maybe afterward you can get her back to your place, maybe not. But the next time you see her, there HAS to be isolation. Ya dig?