@Pvt DeSol
SOI Students can fill in as basic riflemen, which means they cannot fill any role other than Rifleman, Automatic Rifleman, or Assistant Automatic Rifleman. You would have to ask the ACE or Support Elements if they need anyone to fill those roles. In those vehicles, it would basically be as a passenger.
LED is the Law Enforcement Detachment. Same rule applies, you may fill in as one of those three roles if they need it.
The 15th Recruiting Server is our public server. Anyone may use it if they adhere to the rules, and use the required mods (which are different than the 15th standard set). There you can find pick-up groups. If there are a bunch of 15th members on (15th SOC TeamSpeak tag), I would recommend it. That can happen most any evening, usually on Friday or Saturday evenings.
Here's how you should make contact to politely ask to fill in:
Get yours mods in good working order. Test them in an official server. Make yourself loadouts for each of those three roles. You can refer to the Unit Policy Manual, Section 16 for detailed loadouts, and build them in Virtual Arsenal.
Sure fire method is when someone posts in Global that they need fill ins. You can then feel free to join their channel and ask verbally.
Another method is to open a text chat with the owner of the channel 15 minutes prior to the training event start. DO NOT POKE! NEVER USE POKE ON A STRANGER!
For example: 15 minutes prior to Archer 4-1's Section training this Saturday night at 1900CST, you send a text chat to the owner of the channel where it looks like everyone is gathering.
Ask, "Hello, do you need any fill ins?".
They will politely, hopefully, reply, "Yes, good sir, we would very much like to have you fill in with us in Archer 4-1 this fine Saturday evening at 1900CST, for we are a happy bunch and enjoy the company of all willing souls."
Join the channel, follow directions, and have good time fun!
Hope that helps!