I'm not easily offended, so I don't think the actual question was tacky, but how she asked it draped it in tackiness. When I get invited to places, my response is almost always, "That sounds great! Who else is coming?" I don't think that's rude because I don't ask it skeptically, I ask enthusiastically. I'm genuinely excited and interested in who I'm going to see there (and surreptitiously getting a gauge of how many people I'm going to know).
Now, if Diva #3 only wanted to know because she was trying to see if she would need to stay home based on the guest list, she could have asked in this way: Sen probably wouldn't have been offended and never would've been known what happened if something "suddenly came up" and she could "no longer attend." Instead, she kind of implied that that's exactly why she was asking...well, I think she has right to ask that question. Just like Sen would have the right to say his guest list is none of her business or uninvite her if he feels like it.
ETA: Or you could just hand out a disclaimer. I just got an FB invite to my friend's birthday and you could do what she did:
I'VE DECIDED TO GATHER ALL MY LOVELY FRIENDS AND PARTY AND DANCE THE NIGHT AWAY!! IF YOU ARE INVITED ITS BECAUSE I LIKE YOU AND HOPEFULLY YOU ALL CAN MAKE IT TO MY BIRTHDAY CELEBRATION! IF YOU CAN'T COME, THEN NO WORRIES....I'LL STILL HAVE A SHOT ON YOUR BEHALF!!!!! HOPE TO SEE YOU THERE!!
***IF YOU ARE BRINGING SOMEONE TO MY PARTY...PLEASE NO GHETTO PEOPLE, HOOCHIES, OR PPL I JUST DON'T LIKE****