You can interrogate people two ways.
Get close behind them so they drop their gun when you aim at the back of their head, then you can press LB and then move the right stick left and click it in on any of the options, if it's a normal soldier it should probably be "Spit it out", "Tell me where are the others" or something along those lines, special enemies will have different lines.
The other way to interrogate is to grab someone and then press LB, and do the commands with the right stick.
You can only do each person once but for each interrogation choice they have will count as it's own 10%.
Basically find five different soldiers and then perform both interrogation choices.
It's possible this can be done on your own soldiers at your base, if someone can confirm if this works or not would be much appreciated.