How to get into Deadlock
Free-to-play, but not open: access is invite-only during the playtest.
Invite-only, via a friend
Deadlock is free-to-play but invite-only: you join by accepting an invite from a friend on Steam who already has access. Valve has, at times, loosened this — check the Steam page for the current method.
In short: there is no public download yet. The most reliable route is a Steam friend who already has access inviting you from inside the game. Valve has eased the requirements at points during the playtest, so the exact method can change.
Check before you wait
Steam page
The Steam store page shows the current access state and lets you follow the game.
Official site
playdeadlock.com is the authoritative source for any change to how you get in.
Community wiki — not affiliated with Valve Corporation. For the latest and most authoritative information, see the official site.