I see people selling these, weird that they have any value at all.. anyway I have 3 for free anyone can feel free to claim.
2JRGNZ-ZWF7-P2NP2X-8992-EW2NTG
EK2ZPE-H729-JKNX6F-8NYD-CKCHNW
F7NBMY-XBK8-PM4GFM-WDBJ-299ZFN
I'm not sure what they let you do, I know they let you play the game for certain time or level, but don't ask me I really never used any.
I buy copies of D3 every week or so, this is the first time I bought a physical copy and apparently they all come with 3 guest passes you can give away.