First thing to realize is that these are all outside-of-the-official-game, endorsed-and-supported-by-the-official-game, community-organized-and-run events... in short, taking up the charge to pitch-in and contribute while putting your foot out there to begin communicating is the first step. The events all have an organized structure and for the most part you just need to figure out who and where the person is you need to be in communication with to get you and/or your squad settup and ready to go.
As an individual: You can walk-on (don't have to sign up) for FSOs, you just have to first find a FSO registered squad willing to host you by offering you one of their reserved/assigned rides for the night and operation. You can also very much walk-on scenarios, there is a good 1hr to 30min window open before the event's start time that a CM hangs out and simply assigns walkons a country/unit/assignment. The only "rule" is that you show up as a walk-on early enough to recieve an assignment before the event kicks off (and before we're all beyond the point of balancing teams, filling in single pilot slots, and we're all preparing to launch with our groups and carry out our missions on our objectives), so showup over 30 minutes before go-time and ask to be assigned a position you fancy, get an assignment and then go AFK and make some coffee or a sandwitch and come back 5-minutes before go-time to start meeting up and organizing with the rest of your assigned group and group leader. You could also be active in the scenario community and stay tuned for when official registrations for scenarios open so you can register and secure a seat you desire for all the frames of that scenario.
As a group: For FSOs you would want to organize a roster of pilots willing and able to participate for a month. You then register your squad with that roster of players. Difference between FSO and scenarios is that the number of registered pilots and actual turnout at go-time is heavily managed and calculated... so if say you have 12 pilots on your FSO roster, but only 8 show up each week, then you're going to get some heavy pressure put on you to showup with the 4 other pilots you said you would have (thus enter the walk-ons who ahead of time coordinate and find a squad willing (or in desperate need) to host them in a seat). Scenarios are more loose and open roster wise, if one of your pilots can only make 1 or 2 of the 4 frames it's no problem, we (or your squad) will find some walk-on(s) to fill it out. You also don't need to register in advance, just showup 30-min early on the date to get a walk-on assignement. Registering is preffered though because then you get an assigned seat you're assured of and you can communicate with your team and group in the side forums. Your squadies will each need to register independently for the scenario, but weather it's everyone doing what they want and where they want or if you all want to fly the same plane together in the same group is up to your squad and pilots. Stay active here with the scenario community, findout what the next scenario to be hosted will be and get your squadies interested and agreed on what side and what ride you all want to fly with together as a one-squad single group. When registration opens, get your squadies to register and then begin coordinating with the CO of your side (if you all want to fly together, they will most likely find you a group needing a commander and pilots to fill the roster. Fill it up with your squadies and friends that registered, then fill up any open slots left with individual registered players looking for a desired seat. This process is smoother when coordinated well at the start of registering and assigning groups for registered pilots in a scenario, simply because everything and anything is still available, so you most likely will land all the pilots you wanted in the desired seats you all wanted, with room to spare for others to fill in.
