Google Voice is google's free telephone service. I'm not 100% certain it exists in the UK, but a quick search makes it look like it does. It's safer than putting your real phone number because it's more transient. You could probably also use email, signal, discord, or whatever, but phone numbers are the most ubiquitous.

As to what to write, since you don't know of anything happening, you might have to start the group. Is there something you want to volunteer to do? Litter cleanup, volunteering somewhere, helping people register to vote (ok that's probably a dystopian task specific to the US), or whatever?