Helo, a friend of me from Sri Lanka want to come bangkok but the Embassy in Colombo is asking him to get adress of thailand and a guarentee letter from someone in Thailand.

Some can explain me what it is and how it should work?

Thanks

I think they will want a letter from you saying you will be responsible for them while they are here in Thailand.

This could be include covering their expenses and that they will leave when the visa runs out.

This webpage of the embassy there gives a list of documents requrired for a visa. The are the most stringent I have ever seen on any embassy website. Link: http://www.mfa.go.th/web/2012.php?id=1492&depid=244

You could contact them to find out what exactly is requried. You can find contact info on the website.