I send a thank you to most of my received invites. Sometimes it is difficult and if I have not to you, it is only when I am spending too much time away from what I have to do. I think it is important to be thankful in things like this. When we send a request and receive them it is the kindness of the open networkers who help us to grow our networks. Just a simle, Thank you (Name), (your name). That is all it takes.
I usually type out the thank you not having an auto repsonse mechanism too.
All the best and thank you in advance for inviting me. And If you do not receive a thank you note, please realize I can get busy at times......

Joe Slevin