A formal letter for a membership typically follows a professional format and includes specific information related to the membership application or request. We hope the following sample Written Invitation to CSI Membership will provide a template for your chapter's individualized letter to prospective members.It is our pleasure to invite you to complete the enclosed application for Sigma Tau Delta membership. Learn effective membership renewal letter strategies with guide and samples. Improve member retention! We hope this letter finds you well. Boost your renewal rates with a foolproof membership renewal letter! Submit two letters of recommendation. Complete Interview process with district. Try using our Letters of Recommendation for College or Graduate School.