I agree with DPickering. We rented a cabana this past May and I never got the impression that they pay much attention to the number of people in the party. Of course, if the group is loud and obnoxious then they very well may pay extra attention to you. We had 6 (so it wasn't an issue), but I don't think the attendant ever paid any attention to how many people we had. We didn't get the VIP package but lucked out and got a large secluded cabana with a hammock. There was a ton of room for all of us and easily could have entertained a few more people.
I would bet that if you weren't being loud & obnoxious and they asked who was in the party, you could easily just tell them that 5 or 6 are in your party and the other 3 or 4 people are friends you are just meeting for the day, and it wouldn't bother them. I didn't get the feeling that DC is one of those places that plays by the letter of the rules.