How to send invitation letter to my friend to invite him to my country?

Question

i want to invite my friend to my country, how to send him an invitation letter, can i send him ans invitation letter written by me? or i have to get an official letter from somewhere ?

if you know that, please answer me, the procedure ,

Answer 1

Yes i'd like to know the same, i want my friend from USA to send me an invitation letter, but i read somewhere that there's some official document for that. And how can he send it? Must it be thru regular post? Thanks very much for advice.