The Cab Company can only be bought by Franklin ($200,000).

After purchasing the Cab Company and waiting a while, the manager will call you at some point in the game and ask if you could do some special fares. This only occurs while you are playing as Franklin and not engaged in a mission.

There are 9 opportunities to do so - if you screw up or decline all of them, the trophy can be missed. As long as you don't drive like a complete lunatic and kill your clients, it's a walk in the park!