The sampler lets you attain level 13, but there will be no Auction House access and matchmaking is limited to other Starter Edition free trialers. In one month it will be available to all.
Right now the Guest Passes are only available via someone giving them away, but in 30 days time they'll become open to anyone. Buy Diablo III or befriend someone who has.
For more information on the Diablo III Starter Edition visit the. Dibalo III releases tomorrow, May 15th.