That's either calling them in as backup or just using them regularly(assassinate, marksmen, bodyguard, etc) i don't think it requires being done in a mission, as i rarely used my recruits during missions.
It took hundreds to kill me, but I killed humans by the THOUSANDS!!! Look at me!!! I am sublime!! I am the TRUE face of evil!!!
It doesn't require you to do it on a mission. I was just messing with guards in NY testing out the recruits, there are challenges for using each one two times also, so you might as well get that one while you're at it. I forget if it's brawler or thief club. But fortunately even if you do a challenge that isn't on the list, when you move onto the next list of challenges, it will still be considered unlocked.
"invade some poor sap and jihad bomb him" - LazyKenny