Dear …
On behalf of __(name of organization)__, please accept our thanks to you for accepting our invitation to the __(name of reception/event)__. In recognition of __(names/types of people)__ who have supported the needs of our organization, we __(are/were)__ delighted to offer this __(event/reception)__ to you and to other members of__(congress/the community/etc.)__.
Without the help of all of our great supporters, we would not have made near the inroads into __(type of cause)__ that we have and for that, we are very grateful.
__(Name of organization)__ is an organization that is proud to provide __(describe what the organization provides)__ for the __(type of cause)__. Just recently, we have seen some significant breakthroughs, such as __(briefly describe a couple of accomplishments)__. We know that is thanks to the gallant efforts by __( people such as you/Congressional Members and their staff/etc)__. It __(is/was)__ our privilege to hold this __(name of reception)__ in honor of all those, whose efforts and contributions have made such a tremendous difference to our organization.
Thank you for joining us to celebrate and honor our great supporters. It __(will be/was)__ our pleasure to (be/have been)__ graced by your attendance at the reception.
Sincerely,