What we have done is store all of the music on the Synology, and then import this into individual iTunes accounts on our MacBooks. It's a bit involved, but is more robust and works better than the other attempts at this I have tried.
1. Create a new profile in iTunes to allow you to manage the actual library on the Synology. If you are ripping CDs (like I was), then you have to make sure that the iTunes advanced option "Copy files to the iTunes media folder..." is set. Also, change the iTunes media folder to the location on the Synology that you want to use.
2. Import the music, etc, that you want. Using "Keep organised" will result in a well structured directory of music.
3. On each Mac that you want to actually use the library (if the same one as you used in steps 1 and 2, then create a new iTunes profile) make sure that the media folder is set to eg the default, in your Mac home directory, and that the "copy file to iTunes media folder..." option is *not* set.
4. Then use File>Add to library, and select the Synology directory with all of your media. This will add the media to iTunes, but will not copy it to your Mac (the media files remain on the Synology). iTunes is remarkably good about ignoring imports of the same thing, so if you want to add a new item, you can just import everything again and it will zip past anything that you previously imported.
5. If you want to say rip a new CD, then you open the iTunes profile that you used in step 1, rip it, then change back to the normal profile and re-import everything into your library.
Using this method, the original media files (requiring large amounts of space) are kept on the Synology, and the library itself, which is sensitive to concurrent access, is kept just for you on your own Mac.
Stuart