Recruitment can happen various ways but most militaries are required to issue a formal letter of authorization for military activities to their recruits at the completion of boot camp, training, or the selection process.
Militaries often group together to form different armed groups and militaries taking on civilian survival functions are where terms like police forces, labor forces, and work forces originate from. Selecting a military group or smaller command such as a company, battalion, squadron, or platoon is part of the recruitment process. If someone does not get successfully recruited, they are often placed back in civilian quarters and are not required to engage in hostilities if a conflict is present in their living areas.
Those not selected or unable to complete assessment, training, or selection are usually not considered retired military since they were not able to make it through bootcamp, training, or selection. After the initial recruitment and assessment, there is an extensive and intense training period that can last several years and happens before military service actually starts. Upon completion of post recruitment training, a person must have a retired or injured military status to be in civilian quarters. The exception is if they are on official duty within civilian quarters which is often limited in time and scope. Active military can face tribunal action for being in civilian quarters beyond the scope of time in their authorizations.
Currently, there are armed forces recruitment centers in the US that allow a person to express interest in joining the armed forces. The required age for consideration of enrollment is 16 and marketing efforts can often begin at 15 and 1/2. Prior to that, families who have former military training or histories are suggested to place their children in civilian sports and exercise activities until that age in order to prevent an explicit and immediate denial upon expression of desire to enroll.
If recruitment is successfully completed, and training is started; those in military training are allowed to be targetted by foreign militaries searching for traning centers but the attack must be proportional, this is determined by a tribunal and can result in tribunal action against he aggresor or assaulting forces. The assessment of combat capabilities must also be proportionate to the amount of time they have been in training.
Alternate recruitment is often done through college or universities; however that has fallen out of favor as an official letter of military authorizations can not be issued by the college or university and a military must still conduct their own assessments, training, authorization, and approval process. Often times the schools and colleges specialize in high level brain and physical functioning. While, this may still not meet the requirements of available or nearby militaries, it almost definitely helps with increasing the likelyhood of survival as a civilian in areas with highly disfunctional or non existent governments and could provide good work opportunities in other civilian contexts.
Ultimately, when attending a school that specializes in high level brain and physical functioning, the final step is in visiting a bootcamp and asking what the steps are to join if the person is interested and qualified for military enrollment. There is often an assessment of any required civilian obligations before being allowed to be assessed for enrollment in bootcamp and completion of the selection process.