What's the etiquette for joining in on a jam in a public music festival that has fiddles, guitars, banjo, and maybe bass... but nobody playing a penny whistle? 
Is the whistle considered to be a 'problem' instrument or too loud... so that one should always ask before joining in the fun?  Like if it's a folk, bluegrass, or oldtime session- would they not like a penny whistler to join?
Asking for a friend.