ElementX (main mobile matrix client) has live location sharing that works in group chats. Be aware that any location sharing system that loads map data (tiles) from a server automatically leaks your location to that server. The only way to solve this would be to have all the map tiles required stored offline but I’m not aware of any solution that does this. The simplest way to do achieve maximum privacy is to just have something that automatically shares just the coordinates as a text message at a regular interval then you can open those coordinates in an offline map app.