There are three kinds of sessions:

1. Sessions where everybody knows different tunes and they spend 20 minutes trying to think of what they have in common for every 5 minutes of playing. So keep a list of the tunes you know.

2. Sessions where people go round the circle, with each person getting the chance to lead or teach a tune. This is my favorite kind.

3. Sessions where people play the same twenty tunes all the time. The best thing to do is to remember the names of the tunes and learn them from a book or the Internet.

My advice is to take the whistle and see what the session is like. Frankly, I wouldn't leave an instument in the car in a neighborhood I'm not familiar with.