I was, and to an extent am in a situation like this.
Mate of mine - I've been in two bands with him before - he asks if I want to join his new project. I can't commit to it, so decline. He asks me to fill in at a rehearsal, so they can at least practice the material with a bassist until they get someone.
I went along and had a decent night at the rehearsal room. I get the impression from the rest of the guys that they'd been led to believe I was genuinely interested..they'd love to have me in the band etc. I suspect my friend thought I'd be swayed after playing with them
They did eventually get someone, who about a year down the line is quitting and I'm being approached to 'fill in' at rehearsals and a few gigs. I wouldn't mind if that was actually the case - but I just know efforts to find a permanent player would cease if I was there.
I do take it as a great compliment to my playing, but I'm happy in my current band and don't have time for two.
If you are in the market for a new band, you can definitely treat this as an informal audition. Otherwise, if it's a hobbyist level band, you want to help a friend, it could be fun evening. You may also see it as a opportunity for some dep gigs, or at least to build your reputation. Otherwise - I'd question what's in it for you?