Make iTunes handle albums with different artists

Select all the files you want to combine, make a right click and select "Information", go to tab "Options" and set "part of a compilation" to yes...

This blog article deals exhaustively of this specific topic : http://www.blisshq.com/music-library-management-blog/2011/03/26/five-ways-organize-various-artist-compilations/

To sum up :
- as stated by strauberry, declare all files part of a compilation
- decide of a string that define your various artists albums (something such as 'Various') and set it as 'album artist' tag value
- keep compilation, album name, year and album artist tags identical