It is apparent that there is no single (or even a few) common ancestors for the name Griffith/Griffin/etc. in Wales or elsewhere. The reason is straightforward: Griffith is a common first/given name in Wales. When England forced a change from the patrynomic naming system used in Wales prior to the 16th century, the common practice was to simply adopt the given name of the father or other ancestor. Consequently, John ap Griffith (John, son of Griffith) became John Griffith. Let's say that Griffith had a brother named Howell and Howell had a son named Rhys (known as Rhys ap Howell). Rhys would probably have adopted the name Rhys Howell.So two first cousins, John Griffith and Rhys Howell share a common ancestor.
Since Griffith was so common a given name, there are literally thousands of Griffith families today who are totally unrelated paternally. That being said, we are now beginning to see several groupings of Griffith and Griffin participants in our DNA project who do share a common ancestry.
Hope this helps answer your question.
Robert Griffith Project Administrator Griffi(th)(n)(s)(ng) Surname DNA Project