Can I use frequent flyer miles from other people? How would one redeem these? There are many different airlines. Is there a clearinghouse where I can place these miles in exchange for tickets?"
The frequent flyer member will need to book travel for you, and there are usually delivery fees and taxes. Some airlines blackout holiday periods, but not always as long as it is not international travel. It can be very difficult to try and secure a Thanksgiving ticket on miles. Often seats for this and Christmas travel are booked up to a year in advance. That being said, it does pay to check to see if space opens especially after midnight as any reservations that are set for an auto cancel will do so at the midnight mark.
There are sites that allow limited mile consolidation, such as points.com but there are several restrictions that make it difficult to use and there are additional fees.
Thus the bottom line is that the member will need to book the travel for you. If they are registered online, and provide you with access to their account with their membership number and password, then you will be able to check space online. Just get them to try and book asap as seats on miles are limited to about 10% per flight.