The Killzone: Mercenary multiplayer closed beta went live on July 31st. If you received a multiplayer Beta voucher as part of the promotion via PlayStation Home, please note that your voucher is for the Open Beta in late August, not for the closed Beta.
One of the cool things about the multiplayer is that the single-player will cross-over with it. The unlocks you have earned in single player will be available to you in multiplayer and vice versa. There will also be a bonus for players that play often. The more you play, the more cash you’ll earn, while collecting “Valicards”. You’ll also earn more money for killing higher ranked players. This is an incentive to keep players interested in playing more, and rewarding them even after they’ve hit a rank cap. You can even build up a deck of cards that rewards a larger amount of money.
To keep things interesting, a “Vanguard capsule” will be randomly fired into the environment during a match and gives the player that reaches it first a one-shot special weapon. Everything in the game has a counter though, so that’s good news for the players that don’t reach the capsule in time.
Source: PlayStation Blog