Pending invitations are invitations that have not yet been accepted or rejected by invited collaborators. If you want to remove/re-send a pending collaboration invitation, go to the folder/file and look at the Collaborators panel on the right.  Here you will see a list of all confirmed and pending collaborators.

 

Click the ellipses (...) beside the collaborator's email address.

 

Resend Invite.png

 

 

This will bring up a menu with options to Resend Invite or Remove the collaborator.