Sharing files across Nextcloud servers is as easy as sending an email from a Gmail address to a Yahoo account! Instead of an email address, you use a Federated Cloud ID.
An example Federated Cloud ID is firstname.lastname@example.org/nextcloud
To share a file, enter the Federated Cloud ID in the 'share' dialog. A notification will pop up on the Nextcloud server of the recipient.
You can find your own Federated Cloud ID in your user settings. Give it to your friends so they can share files with you!