Note: This achievement unlocks after completing the mission "deputized" and NOT when Marshall Blake welcomes you to the faction. Kind of odd, and made me think the achievement was bugged but it wasnt!"
Here is what to do:
During the main mission "The Empty Nest" with Sam Coe you will arrive at Akila City.
The city has a hostage situation going on. (For me it was happening right when I arrived). This faction mission is called "job gone wrong".
Complete this mission.
"Deputized" is the next faction mission.
You will be sent to speak to Emma Wilcox.
Ask her about joining The Freestar Rangers.
She will ask you to complete one of the freestar ranger missions. Select one from the terminal and complete it.
Return to her and this will continue the mission.
You will be greeted into the faction by Marshall Blake. Despite him welcoming you to the faction this is not where the achievement pops.
Complete the rest of this mission and the achievement will pop!