How do you prepare encounters on Beyond when you know additional monsters are likely to join the fight? It seems tedious to modify the entire encounter mid battle. I've DMed with Roll20 and it was easier to add or subtract monsters from the fight as I went.

Is there an easy or obvious solution to this? I'm preparing the basement for Secrets of Saltmarsh and don't want to include every monster in one encounter, but I also don't want to make five different encounters for one part of the house. Also the monsters will likely move into the battle as it progresses. Please help.

If you know who/what the late arrivals are going to be, I would add them from the start and simply skip them as you run through combat. That way you can have them start at a set round or other trigger. If you are worried about the players 'seeing' them on the list, it might be possible to give them a condition (invisibile and paralysed?) that may mean they don't interact with the current combat sequence (I haven't tried this, just musing how I'd do it).