The invitation can be re-sent by selecting the email address with "(invitation sent)" on it under Access Management in Studio.
Click the email address and you will see the "RESEND INVITATION" button on the right upper part on the studio and the "See invite URL" on the opposite side
A Pop-up message "Invitation email sent" will appear on the right lower corner as a confirmation that invitational email has been sent to your recipient after clicking the "RESEND INVITATION" button.
Clicking the "See invite URL" will lead you to a message box as seen below. If you're having troubles with sending the invitation over email, you can copy the Invite URL link and send this directly to the person you invited over any messaging application.