You forgot to invite one of your close friends to a grand party at your house. Express your regret using should have.

Hi,

The regret will be -

For you :-

You should have invited your best friend to a party at your house.

To your friend :-

I should not have forgotten to invite you to my grand party at my house.

Here is a sample which might be of your help -

"Hello Bro,

I'm Really sorry to you say about this but deeply regret that I just missed out calling you on last Eve's party because of a lot of hush and bush which made my mind sleep out your name.

Beg your pardon, and hope that we could meet soon sometimes later."

Hope this helps.