At least in the above scenario, it was Andres doing the work of initially getting someone up to speed and then pairing the coworkers on an appropriate level feature/bug. He'd stop by and check in to see where they were stuck and would genuinely try and solve the problem.

So, it has to be someone who is a great coder for that stack and enjoys teaching.

If it's done right, other members of the team become good teachers as well, so the initial burden of teaching is lightened.

Pay them well, and give them a regular chunk of (paid) time to work on something that furthers their growth / is intellectually stimulating / whatever.