Note: If you are a Stack Overflow for Business customer, we highly recommend you enable SSO instead. SSO is not only more secure for your system, but it will also be faster and cleaner for your coworkers to gain access as needed.
Unlike Email Auto-Join, invites will work no matter what the user's email address is. Whether this is because your Team is organized around a group that does not have a shared email domain, or perhaps you need a user from outside of your organization to join the Team, invites will allow anyone access in a controlled fashion. As an Admin is required to send invites, this allows you to grant access to individuals for your Team without leaving the door wide open to anyone and everyone. Invites work even if you have Email Auto-Join enabled, though this is not available if Single Sign-On is enabled.
To send an invite to a user, an Admin of the Team can use the left hand navigation to head to 'Settings' -> 'Manage users'. There, they can click the 'Invite' button located in the top right corner to pop up a text field like below:
Enter the email of the user you'd like to invite, and it'll be sent off - the field even allows for multiple emails at once, if there are multiple users you need to invite at once. Note that if your authentication settings include Email Auto-Join, there will be an extra prompt when sending emails to users outside of your approved domain, just as an advisory.
Once you've sent an invite, the recipient will receive an invite link by email that they can use to gain access to your Team. The user can choose between using their own existing Stack Overflow account, or creating a new account, as needed. Once the account is confirmed in either way, they will then be granted access as an active user of your Team.
You can review the status of your invites currently by checking the "Pending" tab on the table of users on the same page. Here, you can review when invites were sent, and also perform additional actions by hovering over their Member status on the rightmost column. Did a user claim that they did not receive the invite? You can send them a new invite from there. Did you accidentally input an email incorrectly? You can remove the invitation prematurely, preventing that invitation from working and allowing an unintended user to hop into your Team.