Not sure how much longer this will be around, but I basically added 2 additional free lines to my T-Mobile One plan today (just paid for SIM cards).
Currently T-Mobile has a promotion to add 2 lines for the price of 1. The way it works is it adds 2 lines for $20 each, then gives a statement credit for $10 per line every month (as usual, may take a few months and a bunch of tweets / phone calls to get the credits working properly).
Now, T-Mobile also has the Kickback program, where if you use under 2GB on any line (from the One plan), you get "up to" a $10 credit for those lines as well.
$20 -$10 -$10 =$0 (as long as I make sure the lines use under 2GB monthly).