That sounded more like an audition and not a Jam.
I help run Jam days for another well known guitar forum - I’ll learn about 20 bass lines preagreed and play them on the day a couple of times through. We get ability range from total beginner to very competent (one guy did a cracking job of the guitar solo in Beat It)
At the end, without fail, I get 20 or 25 guitarists who are very happy and all personally thank me for coming along, playing, and putting up with their playing. We usually have a good drummer and a keys player so it always holds together.
It’s all about expectation I think...