There’s an app I’ve never used but heard of: https://adresilo.com/

I think the way it works is that it queries Google’s API for locations, proxies through them, but it can’t show them on a map due to Terms of Service, but it can provide links that will then open in any map app of your choice.

The app is open source, but the DB is obviously Google. So it’s trying to blur that line of providing Open Source functionality, and using Open Source maps, with the practicality of “Google has all the stuff” UX problem.

At least I think…

XMPP/Jabber via a web client like movim.eu sounds like it ought to work!

You can also look into Snikket as a host for small groups like friends or family, but can continue to use the Movim web client even if you’re hosting with Snikket rather than Movim itself.