On Mastodon, you can manually choose Fediverse accounts to recommend as follows for new users when they sign up on your server. The recommendations can be accounts on your server or on other people’s servers, all you need is their full Fediverse address.
To set your server’s recommended follows, log in through the website with your admin account and go to Preferences > Administration > Site Settings > Recommend these accounts to new users. Paste in the full Fedi addresses of the accounts you want to recommend, separated by commas, and then press the Save changes button.