You know when survival mode starts, in the first few days there's a chance a family member survived and then came in the bunker? what happens?

Does the run get invalidated or does it transform into a 2 member run, what if its already a 2 member run? what happens?

The only thing that matters is how many members you take with you into the shelter before the bomb drops. So if you took 1 member (in other words, if you for example played Ted and took nobody else with you), and then open the door and it's a family member that survived, it's still considered a 1 member run. If it's already a 2+ member run nothing changes, it just stays a 2+ member run.