Can I pm multiple people at once on here? As in a group message for when I host a private game?

No, this option is not available. Unfortunately, you have to send a pm to each person separately

ah too bad.. would be nice. okay well thx for the quick answer!

You would be better off not sending multiple people to a private trade game at the same time, can cause confusion.
I like to make a new private game for each trade to keep scammers from rejoining or guessing your trade game.