This will work differently for different games. For a lot of single-player and co-op games, you will be let in immediately. For multiplayer games, they usually want everyone in at the same time, so you will be notified by email when the next playtest is. However, it's impossible for me to test how these tests work without volunteering and seeing what happens, so no guarantees....NOTE: if the playtest is listed as a demo, I'm not going to list it.
store.steampowered.com
MMO shooter that looks pretty cool. Sign up on Steam and wait for the email. Should be another test soon
store.steampowered.com
Open world, single-player, co-op, recruit animals and build a town, Sign up on Steam (may get immediate access)
store.steampowered.com
You do things, so it's a Souls-like. Sign up on Steam for immediate access
store.steampowered.com
Balatro-like (sort of), Get immediate access that ends tomorrow
www.soulframe.com
By the makers of Warframe. Sign up on the website for a weekly chance to join the playtest
store.steampowered.com
Boomer shooter. Sign up on Steam (may get immediate access)
store.steampowered.com
Engineering shooter. Sign up on Steam for immediate access

PIONER on Steam
PIONER drops players into a vast, grim world, where survival hinges on exploration, crafting, and smart resource management. As an MMO FPS, it offers both PvE missions and intense PvP zones, with unique crafting mechanics, hundreds of weapons and a deep story to explore.

Animalkind on Steam
Animalkind is an inviting open-world town-building game where you play as an intelligent, aww-dorable animal who discovers an ancient mech. Use your powerful mech to establish your ideal town and recruit a variety of characters to join you. It's the paw-fect game to play with up to 3 friends!

Memoirium on Steam
Memoirium is a retro soulslike set in a chaotic and crumbling realm of dreams. As a Dreamer, you must adapt to the world's odd rules and defy others like you in order to escape the strange and eclectic subconscious landscape.

RACCOIN: Coin Pusher Roguelike on Steam
A coin-pushing dopamine machine. RACCOIN is a roguelike deckbuilder where you combine special coins with powerful items to trigger wildly satisfying combos. Stack shiny towers, spin the lucky wheel, and shake the machine until all the coins drop.

Home - Soulframe
A massive new undertaking from Digital Extremes. We hope that you’ll continue following us down yet another new and exciting path to where our imagination brings us next.

HMUR on Steam
HMUR is a fast-paced retro-FPS, in the original setting, that merges Soviet aesthetics with whacky madness and frantic dynamics of iconic 90s shooters. Survive a disaster at a classified Soviet facility as you blast your way through crowds of enemies. Speed is your key to survival.

The Maker Way on Steam
Engineer a fleet of rovers, drones, robots or any machine you can imagine. Use your machines to gather resources, discover new parts, build bases and fight hostile government robots in a physics based open world.