Right next to me? That's a strong no for me, even if I'm wearing a mask.
But it's clearly going to be an awkward period. At Apollo's yesterday, I was alone in the back row and there was a couple waiting in line behind me. I could tell they were sort of hesitating like "Oh, can we? Should we?" and while they weren't masked I'm not concerned about myself and there was a seat of distance anyway, so I waved them on. But if I were at Invadr in the same position, I would not have expected to cram into a car with a single passenger, and wouldn't have allowed someone to slide in next to me if I was.
I guess the lesson is "sit in the closest seat when you get on as a single rider to keep this from happening?"
Pre-covid I've had that happen and I never told someone not to take the seat. And I didn't this time, but then I really wished I had. I should have said something like, "nothing personal, but under the circumstances I would prefer if you didn't sit there". Or something. The guy also wanted to strike up a conversation and even fist bump at one point. Like, come on.