Clans can play with bigger player amounts as well rather than only 8v8 or so clanmatches. But overall i think you need different kinds of players for gameplay experience testing, including players that are completely new to the entire red orchestra franchise.Clans are not the only people who will be buying this game though. If you just let clans into the Beta test then they will tell TW to tweak the game to be good for 6 vs 6 gameplay, or whatever the standard numbers of a clan match are. I would think TW wants a beta test to gather opinions of the game from all groups of players, not just clans.
However where i think clans can bring a big help, is that their forcus is on competitive play, and with that have a big focus (especially for top / super competitive clans of any game) on obtaining a perfect balance and removing all the exploits in the game. Aka they are ideal for the polishing a game, and finding ways how bugs can have an influence on gameplay. An additional bonus generally of clans is that a clan is a group of players from the get go.
Another group that generally works really well for testing is modders and mappers. A lot of content creators got a good eye for visual bugs, and technical issues, and understand what things can go wrong in a map and give feedback on things causing for instantance low performance. Often modders unite under communities or mod teams as well making them easy groups as well.
The nice thing about both modders and clans is that a lot of them get to a scarily fanatical point of caring for and getting addicted with a game, and will help in ones devotion to testing. Not to say there arent people equally devoted elsewhere but its hard to stick a tag on them.
Next to that regular gaming communities are probably nice to use as well, like people from this forum. Which primarily bring diversity and counterweighted opinions to 2 groups that are somewhat got a dedicated focus. And with 3 types of groups like that, if you get groups like that from the RO scene, KF scene, ARMAII scene and COD scene you will find a nice diversity.
But then again you come with the biggest issue with all of them, testers must be trusted (and bound by NDA's) especially in early stages of testing so that information can be shipped out using according to plan.
And that is probably the hardest part here, as its hard to find a big group without any leaks, and that is probably a reason why a lot of companies probably do the final testing as a sort of super near open big beta. So you can shorten the time on the final testing due to massive numbers, so it doesnt that much if people share a bit of info here or there. Especially if compatibility needs to be tested the more people the better so in that sense just give every preorder and website some beta keys .
EDIT: LoL while i was typing this Apos wrote about nearly the same thing Although differently from Apos i think you definitely need regular players especially for input on how they experience a game. As over 90% of the people actually playing a game end up being regular players and they are where the money is at in the end.