Though I’ve played other instruments in groups, I’ve rarely played whistle with anyone else and now I might end up doing so more often. Are there any general guidelines for ornamentation (if everybody does something different, it probably sounds bad), style, etc?
By no means! I mean, unless you get too elaborate. Same goes for variations- try to keep with the group but don’t be afraid to do some things on your own. Sometimes.
My previous experience has been with I guess more structured ensembles like concert band. I’m not used to this idea…
Everybody doesn’t have to be playing the exact same notes to sound good. Now, if you get everybody playing a different tune, then you’ve got a problem. It’s a good idea to play along with some recordings of tunes you know before going, and BBC’s virtual session is especially good for this. And if at all possible, position yourself by a uilleann piper so nobody will hear your mistakes 
Well, the principles are similar. I mean, if everyone did something completely different, of coruse that’d be bad; but a bit of variety is always good. Also remember that different instruments use different ornamentation, and most ornamentation is so quick it doesn’t come through very clearly in a group setting anyway 
(and yet, without it the music somehow sounds dry)
I follow these rules: When in doubt, don’t play. When you know the tune dead cold, keep it simple at first. Play a tunable whistle and keep it in tune. Little ornamentaion differences are great and expected, and even small mistakes go unnoticed mostly, but an out-of tune whistle is a sore thumb sticking out.
i would think it depends on which type of music you’re playing. there are rules to itm that i don’t know anything about. i bet other forms of music have rules too. i like folk music. go with the flow and blend. you won’t know til you try. also, i’ve run across a few songs that we all thought would sound nice on a whistle but they didn’t. be prepared for that too.
Pallas Athena,
I would think the goddess of wisdom would not have to seek advice from us mere mortals on the chiffboard. Perhaps it’s some sort of test.
My experience agrees with most of what’s been said here.
The biggest thing in common with concert band music is use your ears. Love the sound of the group more than the sound of your own instrument. All that sort of thing.
A good deal of the “texture” of Irish trad music comes from the different instruments playing slight variations (meaning different ornamentations) on the same tune. If you hear a bunch of musicians playing a tune in unison without any ornamentation, it just sounds boring. The ornamentation, skillfully and joyfully applied, is what gives this music life.
Listen, join in on tunes you know well, and have fun!
Tom