There are three ways to invite new teammates to your Spacetime team.
Using your invite link or via email
Select "Add Teammate" from the page to bring up the invite dialog:
In this dialog you will be presented with two options: a unique invite link that you can share via Slack, email, text message, or any other medium.
Alternatively, you can have Spacetime send invites via email for you—just enter a list of email addresses you'd like to send invites.
Automatically sync with Slack
If your team is signed up with Slack, you can enable automatic syncing with your Slack team or a specific channel in your workspace. From your Settings menu, select Team Settings.
On that page you will find the sync option:
Toggle the feature on to enable automatic syncing; you can then select whether to sync your whole team or only individuals that are in specific channels.
Note: The sync does respect the limits of your account and will not send invites out once you have reached the team size limit for your plan.