To the OP, while you may need to make multiple reservations to add new people to your traveling party, it is also possible for airlines to link reservations after they are made so the system recognizes people are traveling together.

For example, as a mom flying with kids, sometimes I would book myself and the younger child, then later buy a ticket for the eldest after a school scheduling question was answered. I still wanted to sit with my kids, especially when they were young! Once I learned separate reservations could be "linked," I worried less about having two separate bookings.

On my usual airline, Alaska, I can even link the tickets myself after purchase by going to the "Trip Details" and then entering the other confirmation number. Here's what that looks like with that airline.



My spouse and I virtually always have separate bookings, either because his is paid for by work, or because I want to take a few extra days on one end of a trip or the other and he does not... Separate bookings is usually not a problem.

I hope that helps!