If you've still got the option ticked to 'Keep ITunes Media Folder Organised', select the offending tracks in iTunes, press Ctrl+I (Win) or Cmd+I (Mac) and change the Album Artist to that of the major artist. Ie if the main artist is 'Albert Postlethwaite' but there's a couple of orphaned tracks by 'Albert Postlethwaite ft. Mavis Shufflebottom', change their Album Artist to 'Albert Postlethwaite'. iTunes should then put them back with the main album.
One of the first CDs I ever ripped two/three years ago wasn't a compilation album but did have a song on it from a 'contributing artist', and having seen that by default iTunes wanted to make a new folder for just one bloody track even though it was clearly from the same CD, I learned very early on to be careful with this. Always untick/uncheck the option 'Keep iTunes Media Folder Organised', and when ripping a CDs, unless it's a compilation album, always enter the main artist in the 'Album Artist' field of all the tracks before you import by pressing Ctrl+A or Cmd+A followed by Ctrl+I or Cmd+I and completing the 'Album Artist' field. This will force iTunes to keep all the tracks together like they should be.
Quite why some clown at Apple actually thought we wanted ripped albums split-up all over the shop every time there was a contributing artist on a single-artist album is quite beyond me. I'd love to meet that person ask them what exactly were they smoking.