In order to submit excuses or guest information for an event, it is easiest to be in "List View" so you can see what events require excuse/guest submission. 

Once in "List View" you will see that some events have an 'e' and/or 'g' next to the event title. 

The 'e' is to submit excuses. The 'g' is to submit guest information. 

To submit an excuse for an event you can't attend, click the event and explain why you can't attend, 

and attach a picture to verify if necessary.