Not sure it helps but I was decades ago on a bank sponsored work visa.

Invitation from US citizen is no problem.

Unless she is serious educated, strong roots in Thailand and financially secure hear it ain't going to happen.

Quite right. The main criteria is that she has 'strong ties to the community' in Thailand, i.e., long and verifiable work history in 'respectable' field, land ownership, consistent savings commensurate with salary, etc. Squeaky clean. The concern of Consular officer will be that she will become a 'Robin Hood' in the US and stay illegally. Before meeting the Consular official, she will probably be interviewed by a Thai consular employee, who has a nose for irregularities like a bloodhound. There is no such thing as 'sponsorship,' re. tourist visa.

The US does not require or except letters of guarantees.

If you live in the USA there is no chance

You live here you have chance

Agree with BikerLou

Unless she is serious educated, strong roots in Thailand and financially secure hear it ain't going to happen.

If you live in the USA there is no chance

You live here you have chance

Not necessarily. My "gf" just got a tourist visa and she has no identifiable job, I live in the U.S., and little income besides what I gave her. That said, we were in a unique situation and you can read up on it my earlier posts, but I think they would have given her the visa even if we had a conventional relationship.

Bermuda -- no problem for Thai citizen up to 6 months... your family and friends you and her there.