I found quite a few methods to get people to join.
First of all I usually ask my principle to do my daily recruitment message for my regiment. It usually goes along the lines of, "Hello everyone, I run a hardcore regiment with strict discipline and standards. We use PTS in all of our events and offer an artillery company in addition to our line." This usually gets around 30 recruits a week as people are entranced by the idea just like in any normal high school.
I also find that sending smoke signals is a surefire way to get people to join. The small minority that reads smoke signals will do anything to hang out with fellow smoke signalers.
Finally, I find that going into other regiments teamspeaks and then publicly recruiting in there will get a great deal of members. Of course I always use a VPN as to not get banned on my main and I also use a voicechanger to cover up all loose ends. To date I've been able to sway over 100 members from other regiments.