Bringing SQL Server filegroups online
John Huang (MVP) outlines an approach in his blog post:

http://www.sqlnotes.info/2013/05/07/attach-database-with-missing-ndf-file/

The basic approach is outlined by John Huang as:


Create a empty database with the same file layout, including names, filegroups, location, etc, everything but data
Take the missing files offline
Take the database offline
Use the data files you have saved to overwrite the empty data files
Bring the database online


This allows you the chance to cobble things back together, but it can still have issues.  

So, please, make sure you maintain good and complete backups.