It is rare that you will find any club that has a grading system for Mixed Martial Arts. Although every MMA club has different ways of teaching, most only sort individuals by weight and known ability.
Mixed martial arts practitioners usually have previous experience with other styles. These other styles will most likely have a grading / belt ranking system within them. The respect of a certain grading level that each practitioner has from any martial arts style is transferred into Mixed Martial Arts clubs. Despite this, they are not converted into any single MMA grading or belt ranking system, purely because no person can ever argue that one style is better than the other.
Mixed Martial Arts takes a wide range of techniques, strategies and movements from any style or ideology that has existed and combines. Depending on the size, strength, agility and mindset of a MMA practitioner, the wide range of factors taken from other styles should be combined in a unique way that suites the individual. For example a strong, heavy person may be more suited to grappling, and thus a previous black belt in Brazilian Jiu Jitsu would be much more respectful than a black belt in Kung Fu. In contrast, a lighter but faster and more agile person may not find a black belt in Jiu Jitsu as useful as say the same in Muay Thai Kickboxing – where he/she can utilize the fast kicks and powerful blows to good effect.