If a new user you invited to your Myrmex account has not yet joined the system, you can always send him/her the invitation again.
Simply, proceed as you did to invite him/her the first time. The system is clever enough to recognise this is an already invited user, and will send the person a new email with the link, without duplicating the user.
You may want to warn that person about the email sent by the system so that he/she pays attention to it, and checks if it did not land in the Spam folder of his/her mail tool.