The other thing I will add here, is, as far as I can tell, with some notable exceptions aside, bass players, in general, seem to be “quieter types” who don’t necessarily like being the center of attention (maybe I am just projecting), so I don’t think there is a “rock star” expectation as you would have with a lead guitar player. And while you are ahead of me in terms of getting a jamming opportunity (so take by advice with a grain of salt), I would say to remember that its more important to hit the roots in time than it is to slay a killer bass line with accuracy. It you get flummoxed, just go back to simple roots and fifths and keep the groove going. No one is going to notice if you aren’t playing technical. They will notice if you are not in time.
Final thought: You say these dudes are self-taught, right…so they are gonna be focused on their own parts. Its not like you are going to have a maestro with a full score watching and stopping the jam because you didn’t play something exactly. You will know more about the bass part than they do.
Take for instance Ghost Square Hammer. Lets take worst case scenario and your anxiety is going, your heart is pumping and your fingers are all sweaty and being like jello and you can’t play the main riff. Well, that main riff is just doubling the guitar part. Most people probably dont even realize the bass is doubling that part. And if you can’t play DA-nana-Da-nanna- Da-nanna Da-na-nanna, then just play DA–DA–DA-Da. The important part of that song is hitting the chord changes and chuggin the verse 8th notes. So, I guess, my point is, you will know more if you mess up than they will. And you won’t have Tobias Forge standing over you yelling that there is a pull-of at the end of the main riff instead of a pluck.
Just keep it basic til you get your bearings.
YOU GOT THIS!
Congratulations!
I am aware of what you probably feel now (often when I read your comments I think to myself: it’s so much about me). I struggle with social anxiety.
But you have nothing to lose. At worst, you’ll just say “thank you” and go back to your place, nothing worse will happen. And you can get a nice opportunity for further development! It’s so exciting! I’ll keep my fingers crossed!!
Hit the root on one - Bootsy Collins
Every interview I’ve heard from an accomplished SBL tone is how bad they sucked when they started. And they laugh about it.
Embrace it. It’s part of the process.
Better to suck now than suck 10 years in I suppose.
Every, EVERY, musician has had to go through their first “jam” rehearsal. Every, EVERY, musician was nervous beforehand. So, the people you are going to play with know exactly what you are feeling. Go. Enjoy. Have FUN! After you hit that first note, you WILL BE OK! It is a “right of passage”, so immerse yourself in the experience and revel in the joy that is the reason we are bassists! 
Towards the end, Tom gives some pretty good advice on being nervous
Thanks. It’s good to hear im not the only one who gets the sweats
Love thy bootsy
Youse guys are all so cool. Thank you. Seriously.
Without knowing what they have for a setup, i bought a little amp i don’t mind lugging around for this. If It’s not enough, I’ve got a backup plan. But taking my hartke hd75 would be a drag. It’s heavy, I’m old, and i got zero roadie friends.
After 15 minutes of de-hairing and wiping down it isn’t bad. Plugged in and played it a little. I figure as it was totally worth the whole 101 dollars ( this included shipping!) From guitar center
@autumnsdad1990
This sounds great and so familiar.
As has already been mentioned, it would be great if you could get a lead sheet, or at least the chord progressions, ahead of time to practice.
Nobody is going to laugh at you and you will be amazed at what you pick up from other like minded people.
Please let us know how it goes.
Oh one last thing, if you don’t already have them, INVEST IN SOME GOOD EARPLUGS.
YOU CAN DO IT!!
HAVE FUN!!!
Two things: let them know walking in that you’re nervous about it and that it’s your first time playing with anybody. They should go easy and it doesn’t sound like they’re world class musicians either, which is a good thing. And also afterwards tell us how it went
Already had the earplugs. Being a construction electrician, I’m pretty protective my hearing. And I’ll post after and let folks know how it went. Hopefully. I’ll keep up enough to get asked back.
One piece of advice I forgot to mention is that if you make a mistake, and you will at some point, just keep going. We all make mistakes.
Keep the rhythm going
I’m jealous @autumnsdad1990. I’ve been jamming with some friends but they all play bluegrass. Which is not what I listen to. But I show up because I know it’s good for my playing.
If it’s a disaster you’ll have a funny story to tell. So no matter how you slice it,you win.
I’m pretty sure the story will be funny. I’ll post about it. Hopefully it won’t be too bad. I don’t care what we play, i really just like to make low-end noises.
My suggestion to you @autumnsdad1990 is head along with no expectations on yourself, remember Josh’s words, roots and fifths, hit the first beat, watch for the key changes and feel your way through.
But most important “Have Fun”, and dont forget to keep us in the loop on how you went,
Cheers Brian
PS: you could upload a video 
I’ll get some vids as we play.
Good luck @autumnsdad1990 .
The first thing I thought of was the Bootsy quote @Wombat-metal posted.
I think the lead up to actually going and not knowing what to expect is the worst