How to send a post to a different site and user in multisite?

Hello, I'm building a multisite network where each user has its own site.

I wonder if there's a plugin/solution to let a user send one or more of its posts to another user (thus on a different site) in the network and let the target user be notified.

So far I've found the broadcast plugin, but it' s not fully matching the functionality I'd like to build.

Many thanks for your hints.