Likely the individual/s know they are going to be indicted. Their lawyers are probably negotiating with Mueller or the DOJ right now.
Manafort and Flynn (lying to the FBI) are the obvious targets, but it may turn out to be much smaller fish.
Supposedly lawyers for manafort and flynn said its not them
It is standard procedure for a person under investigation to be notified that a grand jury is investigating them. According to the attorneys of all possible suspects no one has received such notice. There is no reason to believe any of this is true.