If you've ever participated in a beta for a game on Steam, or simply had access to a preview build, you're likely well aware that the whole process is a bit messy. First of all, there is no way to sign up for the beta itself through Steam, so you'll likely have to create an account on some other website and sign up through that.
And then, even if you're chosen, the developers either have to give you a brand new Steam key for a different version of the game, or a password for the beta build which can potentially be shared with others. Either way, it's not exactly an elegant solution.
With that in mind, I am very happy to say that the newest Steam update appears to have finally corrected this. As noticed through the store page for the upcoming Total War: Elysium, not only can you apply for alpha or beta access directly through Steam, but the developers can also grant it on a case-by-case basis and thus completely sidestep the above-mentioned problems.
As an added bonus, this also makes it a lot easier to figure out which games are running a beta test and how you can apply. Besides being nice and convenient, this might just increase the number of participants and consequently the number of bugs detected, which is a win-win situation for everyone involved!
You can read more about Valve's Steam Labs program, as well as the changes to beta access should they get fully revealed, over at the Steam website.