Sources close to the situation tell TMZ ... Brian and Courtney are just friends who met about a month ago via social media. We're told the initial communication was about sharing ideas regarding the possibility of creating music together.

We're told after some friendly chats, they decided to meet up and hang out. And, they did ... hitting up a Mexican joint over the weekend in L.A.

It helps, we're told, they also share similar political views, the kind that makes them want to make a change that's bigger than themselves. For now ... it's platonic, but things could change. That's Hollywood, baby.

Courtney, of course, is fresh off her divorce and at the moment, we're told, she's committed to herself and not looking for a relationship. As for Austin, well, you know all too well.

As we reported ... his marriage to Megan Fox is currently in separation mode. It was clear things were off after photos of Megan and Machine Gun Kelly surfaced last month. Not long after, she appeared in one of his steamy music videos.