This is another easy achievement in "Dark Quest 2"
After your first quest, you will arrive in town for the first time. Once you're there go to the tavern which is located slightly to the left.
In the tavern, you'll be able to hire new recruits. I unlocked the wizard after I unlocked all the other characters, but I assume the order doesn't matter. The first recruit can be unlocked right after the tutorial. To hire another recruit from the tavern you need to complete 5 quests in total. The 3rd companion can be hired after 8 completed quests. The last companion can be hired after a total of 11 quests.
Hope this helps!
Any improvements? Please let me know in the comments below