General admission is usually either standing room or lawn seats. I can't think of a show I've been to that had GA tickets and capacity defined by the number of hard seats, for exactly that reason. I'd believe it could happen, just not commonly.

There are many reasons you might want to split up your group at a GA event, like say some people wanting to push up to the front. Or in your seat example, if your group values being closer rather than being together in further back seats. But that's your own choice and not a result of the venue forcing you.