To invite your friends onto #Mastodon, there's a built-in system for this:

1. Log in on your server’s website
2. Go to Preferences > Invite people
3. Choose the settings you want for your invitation links, then click "Generate"

This often works even if your server has closed to new sign-ups, as admins may optionally allow invitations to still work.

Alternatively, just ask friends to sign up on any server and then exchange account addresses.

Both methods are covered at https://fedi.tips/inviting-people-onto-mastodon-and-the-fediverse/