It totally depends on the venue and occasion, AND the sit-in. I have had many great ones, a few bad ones. I'm sure you've noticed that the good ones don't ask; you have to ask them.
My rule is they have to know the words and the key. NO KARAOKE.
I've been known to break my own rule for a C-note.
Eddie, the same guy has yelled in my ear before! Once it was so bad I had to just stop in the middle of the song and apologize to the audience that I couldn't hear myself. His friends quickly came and got him.
Back in the late 70's/early 80's I did a piano bar (except I used an organ, later a keyboard). It was very lucrative, but a true pita. Never again!
DonM

I refuse to let drunks on stage, even if they are the "hot" girl variety

Another "guest" performer I frown on (except for the celebrity walk on..that we often have during the summer at WW)...folks that bring their own backing tracks on CD or ipod..If you have the talent to "wing" it while I back you live...than I will give you a shot...

Usually I will allow a singer to sit in (vocals), and perform one of our (band) tunes...if they are bombing out..we just take over the tune and fade them out..

Venue is a determining factor...Some places look forward to "regular" guest coming up on stage...

We have a couple great singers, and also a accapella group as regulars..we also have a gal that sings with a flair and doesn't take herself serious..just a fun act..

Many other venues..guest are a no no...and that is okay with me..

Years ago I watched my friends band let a guest guitar slinger/singer sit in. It only took 10 minutes to empty the entire bar, before they could get him offstage. After seeing that, I made a rule that unless the person who hires me asks me to let someone sit in, they can go find their own gigs.

I've always called these "sit ins" as well.

I don't really like when people ask. Sometimes it's good, sometimes horrilbe (either on their part or my part)

The truth is, I can not say "no." So any tips on how to just say "no" to folks that want to sit in? I would feel like a mean jerk saying "no"

LOL, errr, best tip is - NO!! Any more than that invites a response, usually drunken, and then it becomes ugly

Dennis

Just say it is your policy not to bring anyone on stage...so not to slight anyone...You can mention the house policy may be the same..

Yep... always blame the club or restaurant!

'Oh SURE, if it were up to ME, no problem... But the 'house' doesn't want guests!'

Yea thats ok to a point lying to the person ...BUT, ......when they go to the owner and ask & come back to you & say they said it was ok?....then what?

Then you let them sit in. If it goes wrong, you have the club owner to blame! At least he can't blame YOU..!

The cover band I play with always says that it is band policy not to have anyone sit in unless they come to rehearsals before hand. That always puts people off. Of course if we know that people can really perform then we make an exception. It can be very cool to have guest artists join for a couple of songs. But we ask them not the other way around.