Five Grannums have now tested their DNA on Family Tree DNA and it looks like we're all related - we don't know where the relationship starts.
One is descended from Nathan Grannum and Elizabeth nee Jemmott (Barbados)
Another from Arnold Worrell Grannum and Sophia nee Moll (Panama)
A third from David Hugh Parker Grannum and Edith nee Trimington (New York)
Myself and another from William and Ann nee Mullins (Barbados)
Research suggests that Arnold and David both had the same father David Hugh Clifford Grannum but different mothers; there was a third brother Sinclair who later settled in Jamaica.
Further research seems to suggest that Nathan and David Hugh Clifford were brothers born to a Sarah Maria (or Mina) Holder - we haven't found the father yet.
The common ancestor between these branches was from the 1840s or earlier.