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.
If you're already added all your Slack teammates, the "invite teammates" button will be hidden.
Here you can select who you want to send invites to. You can select all remaining users of just pick a few. Once you've made you're selection, click "Invite users". You can add a personal message to the invite as well.