Aside from the public server, you are welcome to fill in for any infantry training (Archer 1, Archer 2, Archer 4). I recommend squad or platoon-sized events. Check the Calendar (under Resources), the Master Roster (under About Us), and squad and platoon TeamSpeak channels to confirm training times.
About 15 minutes before the event starts, ask the squad or platoon leader if you can fill in. Most will say yes! (If they say no, tell me and I'll hunt them down and guilt them)
Side note to any team, squad, or platoon leader. If you see Mr. Cartwright, or any SOI student, in TS and you're about to have a training, say hello and invite them. This IS a social group, after all.