ALA (American Library Association) website:

FRIENDS: A Three-Part Series by Susan Young Gaudiello

WebJunction Resources for Trustees and Friends:
Working Together: Director-Trustees-Friends Best Practices
Libraries Need Friends: Starting a Friends Group or Revitalizing the One You Have
United for Libraries, is a free toolkit focused toward public library Friends groups, but with good tips on membership and outreach for Friends groups of any type.

Organizational Tools for Friends Groups: Bylaws, Mission Statement, Understanding Roles, Policies, etc.