Dear Rich staff for their excellent red pepper tapenade recipe. (It's pretty close to this recipe except without the garlic, of course) By the way, we've discussed recipes before and we think this post and this post address many of your concerns. In general, you're probably fine operating in the manner you described. You rarely need to be concerned about copying one or two recipes from a cookbook. But you should be concerned when a bunch of recipes are copied from one source. In that case, you're more likely to hear from the copyright owner who may claim that their collection or "compilation" has been infringed.
What should your notice say? We think your Terms and Conditions (to which each member of your group should agree before joining), should include a statement to the effect of: "We don't permit the posting of materials that infringe copyright or violate the law and we reserve the right to remove such materials." That's pretty general but it sets the tone in case you are asked to remove materials. FYI, attribution and linking are fine but keep in mind that they won't get you off the hook for infringements.