The Military Recruiter is essentially a mini-Head of Personnel specifically assigned to recruit more Marines into the available positions; one of the few things the Head of Personnel has no experience with or power to assign. Unlike the Corporate Recruiter, you are looking for new recruits from all walks of life to fill out the squads. If none are needed, you can always recruit Military Cadets with dreams of combat. You will likely work in tandem with the Senior Enlisted Advisor, if one is present, to get new recruits into basic training.
When the military needs more personnel, you're the one to get them to sign up. If the military is truly lacking in crew members, you are expected to take the initiative to hire more civilians! Offering pay raises, reimbursements, or other sign-up bonuses are good ways to ensure that new troops will always be ready to replace any significant losses.
Once you've got someone interested in joining, it's up to you to determine their fitness for duty and get them to fill out their paperwork. You may opt to have the local Psychologist provide insight on their mental state, or work up their mental fitness chart along with their physical fitness chart. (Basically, this is both for RP reasons and security reasons; you don't want a troublesome civilian handling the howitzer, do you?). Make sure the new recruit knows what they're going to be doing before assigning them to a position, especially if that position is not Cadet.
There is a military recruitment form to use, which you can add your notes and recommendations to once the future jarhead has filled it out. Keeping a record of all of the civilians you've recruited is key to keeping the Executive Officer informed about what you've done and keeping you out of trouble.
Finally, when everything is signed, stamped, and filled out, you can sign the form yourself. Ask the crew member for their Identification Card so that you can officially assign them their new job title, rank, and access.