Transfer logins to another server
You will want to use the 'transfer logins' script provided by MS.  Pinal had a write up of it here.  I do not remember if I used this to move logins TO 2012, I think I did to setup a QA environment but not sure.  You might need to make minor adjustments but I think it'll work fine.

Keep in mind if you use AD logins and changed domains with no trusts, those users will need to be recreated with the correct security permissions which this script does not do.  This post references SQLSoldier (A SQL guru) and has a handy script.