This article is directly related to Accountants and Tax Preparers, but other professions probably can use these groups to get referrals, too.  See the details in this AccountingToday post by Jeff Stimpson:

http://www.accountingtoday.com/news/tax_news/reference-material-getting-referrals-from-other-professionals-71624-1.html

I have gotten referrals from a group of other professionals that were in a referral group with me.  We all left the referral group, but have stayed in touch and still refer work to each other.  Financial planners and advisors, IT consultants, mortgage brokers, insurance brokers and attorneys give me most of my referrals, but I also get some from entertainment production companies I have as clients (those are more client referrals than professional referrals, but I will take them all.)

 For financial,  accounting and tax musings,