Find where you save the library. Its a *.sldmat file.
Or Create New Library and copy material there and share the file.
be sure to save them somewhere not on the pc. nothing like going threw all the trouble to make the materials then to forget to copy them and then you upgrade and boom they are all gone.
then you start looking at doing them again.
That or someone makes a change to something like the density and all your parts have gone on a diet, and lost a lot of weight, or gained a lot depending on which way the # went.
So keep a back up
As Frederick Law stated above. If you and your co workers are using a network drive make sure your material library is on that network drive and your co-workers are pointing to it for custom materials
Find where you save the library. Its a *.sldmat file.
Or Create New Library and copy material there and share the file.