Once you send out invitations to suppliers, these are marked as Invited on your My Suppliers page in the Connection Status tab.
From now on, the invitations sent out will also show the indication :Invite expires in x days
If you would like to modify the invitation in some way (for example change the contact email address), you can click Edit invitation.
If the invited supplier doesn't react to any of the invitation mails after 30 days, their invitation is considered expired and they will be marked :Invitation expired
You then have the option to resend the invitation by clicking Resend invitation.