You can use Google Maps app to mark a spot and share it. As Paul stated, you’re not going to be able to reliable pinpoint right on top of a rock with the GOS accuracy on a phone, but if you’re marking an object large enough to see if you’re within 20’ of it, it would work ok. In the Google Maps app, click/hold on spot where you want to mark a spot (called Add Label in Google Maps), a box will pop up where you can enter a name (Rock, tree branch, etc.). The you can share the label with people.

You say you don’t use Climate. If you did, and had all your people logged into it, anyone could drop a flag, and even take a picture at the location of the flag. This would be shared automatically with anyone logged into the account.