LEGO 2K Drive

A fast way for doing this I found is the following..

Play story mode, start saving up Brickbux. Cheapest characters are 2000 each, then 4000 & 6000.

Purchase the characters from Unkie's Emporium. Go back into Story Mode, Click Pause, open Loadout, pick a Loadout, go to drivers, hover over each character in the Collection section. That's when it unlocked for me.

You will unlock characters by doing things like races in Story Mode but this will take a lot longer than buying them in the emporium.

Hope this helped!

Cheers!

This can be easily achieved without spending your cash at all.

Firstly you need to complete a race at all possible levels (C,B,A,) - possibly reverse if it exists.
Each race will have a rival. If you speak to a rival after completing each level on a race, you will acquire that driver into your loadouts. The rivals should be very close to the race and can be spoken to by clicking LS in when close.

Easy to complete, can save your cash for buying vehicles / parts you want. If you do every race in the game you will easily end up with 50+ racers just from this.