I think the group that lived in Roanoke ran out of supplies so they sailed away to the nearest place they could find.
A small group of men went out to find a place to live and discovered Croatoan Island. They went back to Roanoke to get everybody else and when the people heard, they carved "Croatoan" on a tree so that John White would know where they went.
All the people went to Croatoan Island and they became part of the tribe. They moved with these Native Americans when they traveled inland.