Not having a seat down rehearsal dinner- What do I write on invitations???

I don't think there's any reason your guests need to know this in advance since you'll be serving a lot of food and they will still be full. Instead of "Dinner and dancing to follow" on the invitation, say "Cocktails, heavy appetizers/hearty hors d'eouvers (but spell it right haha), and dancing to follow" You could also put this information in a details card or on your website.

Also, when you say there's no seating, do you mean that there aren't traditional tables or do you literally mean that there are no couches/seats/places to sit for the whole night? I hope the former since some people won't want to stand for 6 hours straight.

Well its only our rehearsal dinner so there isn't any seating just an "open house". Guest can stop in any time from 6-9pm.

OP said rehearsal dinner not reception.

I don't think you need to state this anywhere. It's nice enough you're including all out of town guests in the rehearsal dinner. Hopefully there will be at least a few seats and hi top tables though.

Also confused as to what "no seating" means - do you mean you're not assigning tables to people? As long as there's enough food to feed everyone a normal meal amount and enough chairs and tables for anyone I think you don't have to mention anything special.

Lyla, she is talking about the rehearsal dinner, not the wedding. I highly doubt people will be there for 6 hours.

OP, I would probably call it a cocktail style open house. You can state finger foods and appetizers if you want to be more specific.

Where are people supposed to sit? Are there stools?

For the rehearsal "dinner", I think you could put something like "join us for cocktail style food and drinks". That would give people the hint that it's not seated

I like @Courtney's suggestion!

Skip the invites and just text the wedding party and explain the plan. No need to make a big thing of it.

Are you inviting any grandparents, parents or elderly guests who might want to/need to sit down?