To invite teammates click on your avatar and click on "Team Settings" in the dropdown.
In your team settings, navigate to the "Teammates" tab. Here you'll see all the users who have already created accounts in Tettra. Click the "invite teammates" button in the top right.
Here you can select who you want to send invites to. You can select all remaining users from your Slack team or just pick a few. You can also invite users by email.
Once you've made you're selection, click "Invite users". You can add a personal message to the invite as well.
After inviting teammates you can set their user role, which grants them different levels of permissions.