"I was there module". (see very poorly done flowchart http://www.mariff.com/modIWASTHERE-flowchart.png ).
Basically, some type of image (you don't have to make but ability for it to be an image) that when a user clicks, a box appears (preferably done w/o javascript or degrades gracefully) with the ability to confirm that they were there with a textbox that they can insert comments in. The user presses submit and an email is sent to the node author. The email should have the username, the nodetitle, and the comments that the person who clicked the button wrote (if any).
Now the tricky part is how to accept. The node author should have the option of accepting, declining, or saying not sure. But he should also in the future be able to no to yes, maybe to yes, yes to no, etc that way if they make a mistake its not the end all be all. So I was thinking maybe a page that a user can track all of the people who say they were there for each story and a simple drop down saying if they were there or not.
Then on the node, it says how many people were confirmed there, how many said they were there but weren't, how many not sure and how many unconfirmed (e.g. the user hasn't responded). So basically under the "I was there image" it would be
Confirmed
yes: List of people that said yes
no: List of people said no
not sure: List of people not sure
pending