I think it might be a good idea to have the transit location or airline name in both the Search for Companions feature / Create itinerary feature.

Here's why:

1). It is a superior experience for your users to be able to search for companions for travel from say San Francisco to Bangalore via Singapore or via Hong Kong or via Amsterdam. If they can only specify the "to" and "from" destination they will have to sift through the results to figure out who is using their exact travel route )including the port of transit).

2). The same can be achieved by allowing the users to include the airline name in their searches (as most airlines have a fixed transit hub).

Good Luck.