I haven't looked into contemporary philosophers who discuss friendship - do you need to be in touch with a current philosopher for the information you're seeking?
If not, then as Hypatia stated, Aristotle's "Nicomachean Ethics" contains some chapters on friendship. Here is a link to a pdf of a translation of the book:
http://mfs.uchicago.edu/institutes/happiness/prereadings/Nicomachean_Eth
ics.pdf Also, this webpage lists some references that you might find helpful:
http://www.markvernon.com/friendshiponline/read.htm