I'd like to allow members to say they are attending an event and bringing their spouse, or their kids. Rather than just say "Going", I'd like to have an option where they say how many people will be in their party. Can you do a +1? Or is there a plugin for it?

I've the same question. Is this possible and I didn't find the way?

There is currently no facility for this within the software, unfortunately. Im not aware of any plugin for this at the present time, but someone else may of course correct me on this