Currently only medics supported. Engineer support is in the plans.

So what it does now:

You call MEDIC!
Medic presses B...
Medic comes to you!

The teleportation consumes the medic's ÜberCharge load, the longer the distance to the client to be helped, the more charge it will cost to teleport. If not enough charge is available, then nothing happens but the medic gets a message telling why his 'B' key didn't do anything.

By default the MEDIC call is valid for 5 seconds, so after someone shouts MEDIC!, the medics have 5 seconds to press their teleportation button to be able to go to this person.

If multiple people have requested for medic, you will be teleported to the closest one to you (assuming Medic point of play in this sentence).

Console variables:
ct_medic_initial (default 0.0) - how much charge is used for transportation (value in %, 0 = none, 100 = needs full charge)
ct_medic_increment (default 0.015) - how much the transportation cost increases per game unit. For example with default settings, teleporting to a player 1000 units away will cost 15 % of the charge.
ct_medic_shouttime (default 5) - time in seconds how long the MEDIC voice command is valid for teleport requests