Franklin will contact you to arrange to meet him at Los Santos Golf Club to meet a potential new client, Dr. This contract is only available after completing a Security Contract via the Agency (you need to purchase one, too).
Dre is in GTA Online, you meet Chop again, you get to listen to tracks produced specifically for GTA Online, and you make a million just for beating the missions. This is where all the fun is: Franklin and Lamar make a return, Dr.