From your HouseAmp account, click on the menu option on the left for the group your contact is assigned to (Home Owners, Real Estate Pros or Service Pros).
Next, if you have any pending invitations for this group, you will see a blue link that says "Show pending invites". Click on this link.
The new expanded view will show you each invited contact with a "Resend" and "Delete" option.
Click the "Resend" link and a new email will automatically be sent to your contact.