Visit the users page.

You will see a table with a list of your company users.

Click on the "+ Add a user" button.

Enter the email of te user you want to invite.

Click on "Save"

The person you are inviting will receive an email with the invitation.

Optionally you can copy the invitation URL clicking on the 3 dots on the right (once you invited a user it will appear on the users table marked as "User has a pending invitation")