If you run your own Mastodon server, you can manually choose accounts to recommend to your server's new members when they sign up. The recommendations can be accounts on other servers, or on your own server.
To manually choose recommended accounts, log in through the website using your admin account, then go to:
Preferences > Administration > Site Settings > Recommend These Accounts To New Users
Paste in the full Fediverse addresses of the accounts you want to recommend, separated with commas, then click the "Save Changes" button.