Depends. I'm of the opinion that you should focus on one at a time. I know people who don't, though. You can RP in others while you GM the one, however.
If Post is readable, ignore the mistakes outright.How do you take care of players with a lot of grammatical mistakes without sounding like a total asshole?
If post is unreadable, suggest they drop their post into MSword
You're the GM, right?And what would you do if you're interacting with another player but the player went inactive?