By contributing you are giving away some of your data to the public domain (typically a title and a date). This means it can be used by others in any way they want.

For now, choose data that isn't sensitive, that can mean a title and a date, but possibly also an image and a longer description. If you want to be more restrictive you can also just publish something shorter, such as the word "concert" and a date. If you're a music venue, it's still valuable with a reminder you have a concert next week.

There is a difference between publishing a book in the public domain and calendar data. In most cases, calendar data is only valuable if it's new. If you decide to stop publishing your calendar data, the existing old data might not have much value. This way, you have bit more control if you're not happy how your data is being used.

Eventually, we might need various levels of licensing (perhaps similar to Creative Commons), various levels of technical restrictions (who can see what), various ways to prove who is who (signing), various retaining policies (how and how long can you keep a copy). We might need all variants from totally open to to totally not open.

However, it might be easier to start open and then gradually restrict, rather than the other way around. The challenge right now is to create a critical mass of data to work with collectively.