Have a recruit reach Level 11 (Assassin).
While your recruits can gain some XP when you call them into battle during your main game, you can likely get more XP from side contracts.
To get to the Contracts menu, Hold
to make the Recruit Abilities screen come up and while still holding
press
to go to Contracts.
This is where you can assign your recruits to special missions so they can gain XP to level up. Each state/colony is difficulty 1 to 3 and has 6 missions available. When starting out, begin with a difficulty 1 state/colony and work your way up as recruits can take on higher difficulties when they level up. Assign one or more recruits based on the %-chance of each mission's success. Once assigned to a mission, those recruits will not be available for times you Call Recruits. So try not to send ALL your recruits away at the same time if you plan on doing any active combat where the extra help would benefit you.
Recruits need 1100 XP to get to Assassin level. After that, they can still do missions, but won't accrue any more XP.
XP for each difficulty:
*** Spoiler - click to reveal ***Difficulty 1 missions are mostly 40XP for each recruit assigned
Difficulty 2 missions are mostly 50XP for each recruit assigned
Difficulty 3 missions are mostly 60XP for each recruit assigned
Difficulty for each state/colony:
*** Spoiler - click to reveal ***Quebec is difficulty 1
New Hampshire is difficulty 1
Massachusetts is difficulty 3
Rhode Island is difficulty 1
Connecticut is difficulty 2
New York is difficulty 3
Pennsylvania is difficulty 3
New Jersey is difficulty 2
Delaware is difficulty 2
Maryland is difficulty 2
Virginia is difficulty 3
North Carolina is difficulty 2
South Carolina is difficulty 1
Georgia is difficulty 1