"Oh yeah, I'll be back in a bit. Just realized I forgot something in my car..." They'll probably not even miss you. Just go home. I would. If anyone asks, you weren't feeling well. It's true.
I have done this so many many times. Just casually end up where there is no one really to pay attention to you or go to the bathroom. Once you you see that there's no one looking at you to see you leave, escape. Listen to your favorite songs on the way home and get excited for what you're going to use your unexpected free time on.
I'm not sure autistic people realize that's an option. My daughter's autistic and I don't think it would ever occur to her than she can simply bail without explanation.
Go to the bathroom and “get lost.” You’re in a strange place. It’s plausible. Slip out a side door and leave.
They mean well, but these things can be such bullshit. These days, I just thank them for the invitation and send my regrets. If they want to make an issue of it, they can be my guest. The fact is, they need my autism (to show diversity and disability in the workforce) as much as I need their job, so neither of us is going to make a fuss.
Leave. If your boss/manager/whatever starts to make a fuss, “I noticed you weren’t at the lunch”, lie and make a fuss right back, “I was in the bathroom chundering my guts out for two hours. Did anyone notice that?” and take the next day off sick with an unspecified illness. Your boss/manager/whatever will be so embarrassed, and unconcerned for your welfare, that there will be no repercussions.
I feel you. Our team potluck (Small team of people I am comfortable with) was canceled. Instead we have to go to the corporate office and have a pizza party with a bunch of people I don't know.