Play whatever you like honestly although It's nice to be ready to play Dire Avenger, Warlock or Fire Dragon. Avenger for when a capper is needed, Warlock for supporting when needed and Dragon for tank busting when needed.
Don't pigeon hole yourself into one class because it's more desired, try all of the classes and decide which one suits you the most. All clans should accept you no matter what your preferred choice of aspect is. Bonus points for being flexible and being able to fill in gaps for what is needed in the squad at the time.
Positive Attitude is the best class any guild needs whether you're in one, allied, or just in the same game as one. If you come with that we can literally work with anything; including playing other races... And forgiving each other when our Elites get blown up in friendly fire.
If it helps, my intention on coming into this game was to play Scorpion exclusively. If you are interested in always being of real value to your team in a match, that's not going to wash. As Xio says above, the team will need Dire Avengers, Warlocks and Fire Dragons for very specific purposes as well as Howling Banshees, Dark Reapers and Swooping Hawks for more general ones depending on the situation. For my part, I almost never play Banshee or Dark Reaper. Banshee because I'm a Scorpion at heart and they're my rival Aspect. Reaper because it's just not in my nature to sit back and camp, so I horribly overpursue, which is the worst thing you can do with a Heavy Weapons class in this game.
It's funny what you say about the reaper, because I played him probably the most aggressively among eldar classes. Reaper launcher you definitely should sit back, but between the IoK, shuriken cannon, and tempest launcher there's a lot of forward offensive roles you can take on. Anecdotally, the shuriken cannon will hold up at close quarters and significantly add to your team's firepower, and the tempest will wreck enemies who stay together. So I guess not only should new players try all the classes, but experiment with them and see what you can do. Scorpions can do more than guard cap points* *I have bashed on SS and SH before, but what I was really bashing on was players who use those classes to skirt around the enemy and sit on enemy capture points calling for a DA who cannot fly/stealth past the enemy. I do not hate on people just for playing those classes, I hate on people who don't help the team.
I haven't spent enough time with the Tempest Launcher to really learn its nuances, but you could well be right. I can't get the hang of the slowness of setting up with the Shuriken Cannon though. Or the Heavy Bolter/Autocannon/Dakka Deffgun either, mind.
It's a mindset like anything else. DA can't just charge in, and melee has to control that distance. I found the Icon of Kurnous makes a difference because you have more control of where the engagement happens. Followed up by knowing where to expect ambushes and prepping ADS early, you can handle a lot of firefights/charges that other factions wouldn't expect an eldar to be able to manage. As well staying with your DA, who if he's playing well will probably flush out the enemy for you, makes for some solid teamplay that's easy to get going in a pick up group. I don't think it's a bad thing if it's not your style though. I struggled to play SS well, and it is/was probably my least played class in EC. I always follow threads about SS just to see what I can pickup.
To be honest any guild will let you play what ever you like and should never require you to play a certain class. Me I mostly play reaper tempest but I also am able to swap over to da and trying out swooping hawks at the moment. But if you want to play one class then that's what you want and you will still be welcomed.
