You can use a self-hosted repository such as gitlab or gitea/gogs. Gitlab is pretty full-blown with commercial and community (free) versions, gitea (which is a fork of gogs) and gogs are very light-weight. I run gitea on my private server for stuff on which I'm working from different development machines. For just 5 developers, it's probably all you need.
Of course, every self-hosted option requires to set up a server that is reachable by all users. This can be either an on-premise server if your company network has static external IP addresses, if not, I'd rent a minimal virtual root server (this is what I have for private mail, web applications, and other stuff.)
I'm not aware of any Peer-to-peer git options, if they exist they are probably not viable between just 5 developers.