Mastodon includes a built-in system that lets you invite people to join your server:
1. Log in on your server’s website
2. Click on ⚙️ Preferences (if you’re on a phone click ☰ in bottom right and then ⚙️ Preferences)
3. Click on "Invite People" at left of the screen (if you’re on a phone click ☰ in top right and then "Invite People")
4. Choose the settings you want for your invitation link and then click "Generate"
More info about invitations at:
➡️ https://fedi.tips/inviting-people-onto-mastodon-and-the-fediverse
