I wonder about the practicality of this, though. To actually travel with someone, you tend to need to know they want to travel with at the time of booking. Meanwhile, booking for frequent travelers is often a fairly immediate or uncontrolled basis (eg, have to balance good fares and upgrade availability confirmable, or award availability confirmable, with an ok from work to take the time off). Trying to factor another person you don't know yet into that sounds logistically challenging.

But, on the other hand, if you wait until later to add them to your travel, you may not be able to (availability of fares, awards, and seats being what it is).

Or if it's work travel, work often books your trip, so how do you add someone else (unrelated to your work) to it?

(That's assuming that it's air travel that's being talked about here. Obviously, the logistics of adding an additional person for land travel may be much easier -- it's just a variation on picking up hitchhikers! -- but I don't know how much interest there is on that site in land travel vs air travel.)