How to only restore a specific filegroup?
A filegroup restore can be done on a database in FULL recovery model.  There are requirements and consequences of the restore.


You must be using Enterprise Edition.
The primary filegroup must be the first restore.
Filegroups that are not yet restored remain offline.


Start reading at this topic in MSDN:  http://msdn.microsoft.com/en-us/library/ms177425(v=sql.105).aspx

Read the topic: "Piecemeal Restore Under the Full Recovery Model"