Experts Global are excellent for GMAT preparations and admission counseling.
To clear the doubt here, they don't a guarantee just like that. you are required to undergo a diagnostic test. If you have already given your GMAT then looking at your calibre they guarantee an improvement of unto 50 points. If you haven't taken the GMAT yet then they provide you a tentative score depending on your performance.
These guys are very smart when it comes to assessing an applicant's capabilities. Very genuine people. I would highly recommend.