a. To begin, click "Configuration" in the lower left side of your screen.

b. Enter "Settings", and then navigate to the "Users & Roles" section.
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c. From here, you can invite new users via email by pressing "Invite users" in the top right.
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...and thats it!

FAQ

Q: The person I invited did not get the mail.

A: Ask the person to check their spam/junk inbox.
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​Q: The person I invited did not get the mail, it is not in the junkmail either.

A: Try to invite them again, there might have been a typo in the email address.