If you read revelations it seems to elude to the idea that the greatcrowd are those gathered during the time of the great tribulation and include those that have died before. Rev chapter 5 i believe tells dead ones to wait until others are gathered then all are standing before the throne in heaven. The 144k are just a group that is part of the great crowd not seperate from it.
Then the book indicates that all these would come back to earth and reign as priests as a holy nation. Lets also be clear that in the original greek in chapter 5 they are not kings and they do not reign over the earth but on it.