As Randy Pausch said, if there's an elephant in the room, introduce it. That normalizes the situation and reduces social tension and confusion. It maximizes the help you'll get from your experienced dancers. I'd also try to set the newbies' expectations that this might not work out so easily, but that's ok and they're wanted. This is long because it's a script...
"Hey, we've got some new folks who just joined us. Welcome! Try to spread yourselves out among us so we can all help you out. You'll get this quickest if you partner with experienced dancers for the first few dances, and dance with the one you brought later, but it's up to you. If you came with someone special and you're most comfortable dancing together, that's ok, too. Also, some folks really benefit from the workshop we give before the dances, so if you find jumping in is a little too much right now, please give us another try at 7pm before the next dance, and enjoy the live music and company as long as you like today. We really want you here! Would a few experienced dancers raise their hands and dance with these fine folks?"
In our group this leads to cheering, and sets the expectation on both sides that people are going to be giving help. One or more experienced couples will split up and invite a newbie into their group or dance as a couple with a newbie couple. YMMV on this. The first 90 minutes of our dance is designated newbie dancing and we frequently split ourselves up to get with newbies. Conversely, the last 90 minutes is "loose the hounds". If your dancers expect to be challenged from the start, you may need a different script.
Then...
"Folks, I'm going to walk this one through a little thoroughly, which should help anyone who is new tonight. Experienced dancers, please stay with me and don't get ahead or give extra advice, but do work with the new dancers to help them get it. And please, let's not chatter. Your investment of patience will bear fruit shortly!" [Again, setting expectations, being inviting to everyone, trying to get them to see the benefit of creating the best learning environment.]
Then, ditch whatever dance you were planning, and call Midwest Folklore, Airpants, Easy Peasy, etc., something with a low piece count, connected, simple figures, simple progression, recovery moves. And yeah, teach the swing. Oof, that takes time. Or, have them do a two-hands-across or elbow swing. The important thing there will be remembering to exit on the same side as they entered. If necessary, progress twice at the start, as was mentioned.
Something similar just happened to us last Friday. TWENTY new
dancers came as a group, an entire social club. We knew some of them
and had been encouraging them to come for some months, but we had no idea they were
coming that day, and our dance is usually only 20-25 people. And, it
was caller Sam Sharpless's second night of calling, ever. They did come for the workshop, and Sam did great with them, there, but still, some of them were...going to take a few evenings of dance before walking on the beat made sense to them. Sam and the gang really stepped up! Sam blew up the first half of his program and called super-easy dances. All the experienced dancers danced with newbies. We joked with our 4th timers, asking them how it felt to be one of the most experienced in the room. Sam gave some careful and very encouraging walkthroughs. It worked, even with often 3 newbies per minor set. The best news is, this group said a few days later that they're coming back next week!
--jh--