How do plus ones rsvp

You should be able to add a plus one onto your original guest from your end. And obviously, when you are addressing the invites address it to "name and guest." You should initially plan on each person bringing a guest, but you will know for sure when they RSVP, and give you their plus one's name.

Usually you can do "and guest" for people so they have to RSVP for two people and fill out the guest's name, I think?

Yes.. We addressed the invitation to our guest. On our RSVP we reserved X amount of seats in the guests honor. So I should just add some wording in the RSVP section on the website about them sharing names then.. I would just put "Guest" as the plus ones name though, i assume?

Understand the struggle.. I'm trying to figure out this question with kids. My FMIL sent me their side of the family guest list with the couples names and 4 kids or 2 kids, but no kids names. SO I'm trying to think of a way to rsvp online without going back to her to list all the kids names