I don't know whether this topic has been brought up or not. If it is, close this.
Shanks and Buggy are led by the same captain before they left to form their own pirate groups.
Shanks and Buggy might be White Beard's subordinates who left shortly after the Buggy's flashback to gather their own pirate group.
In the flashback, they got Buggy's Devil Fruit from an enemy ship. Devil Fruits are extremely rare to come by. A pirate ship having it means that it should have a great power to come by the source of Devil Fruits, if not, there will be too many people in OP world to have Devil Fruits. Buggy and Shanks' ship defeated the enemy ship and got the Devil Fruit. There seems to be minor or no injury/casualities in the ship. It means Buggy/Shank's crews are very powerful, like White Beard.
White Beard said "brings back memories" and "that kid", when he heard Shanks' name. Shanks said "that old man still haven't changed" when he knew WhiteBeard tore away the letter. Buggy seems to be very clear of White Beard's character that he doesn't spare people who murders his crewmembers and Buggy said to Alvida that he "met him on the Grand Line." To get that close with the most powerful man in the world to an extent that they call him "old man" and know very well of his character, they must be the former crew members of White Beard's ship.
Those lines suggest that they probably didn't contact each other at all since they left the ship.
I just reread vol. 1 yesterday. I wonder why Ace never appears with Luffy when Shanks was in their hometown.