Doesn’t seem like it uses ActivityPub, still interesting, and open source
In Nextcloud, user’s Federated Cloud ID works similar to an email address or a Mastodon handle, allowing them to exchange data across servers: share files and and collaborate on documents, communicate in group chats and make audio and video calls.
Federated tools available in Nextcloud:
- Federated file sharing
- Share documents and media to users in other Nextcloud Hub instances for viewing, editing and collaboration.
- Federated chatting
- Create group chats with users from different servers and use many essential chat tools.
- Federated calls
- Make audio and video calls with Nextcloud Talk among users from different servers.
Well, there is a social app for Nextcloud which uses AP, so you can use your account to connect to the Fediverse. That functionality is separate from the file sharing though and not installed by default.