What happens to an airborne plane when its type is grounded by an authority?
You can find the full wording of the EASA grounding order here which does not specify a course of action for planes mid air but does call for the immediate grounding at the time the order goes into effect (March 12th 2019 @ 19:00Z)


  For the reasons described above, pending the availability of more
  information, EASA has decided to suspend all flight operations of the
  two affected models. 
  
  This AD is considered an interim action and
  further AD action is expected to follow.


The AD points to sections 301 and 303 of the EASA regulations which does not seem to shed much more light on the situation other than;


  Any applicable airworthiness directive must be carried out within the
  requirements of that airworthiness directive, unless otherwise
  specified by the Agency.


Which points us back to the AD, which again offers no specific instructions on immediate grounding. 

However according to FlightRadar24's Twitter they are seeing some diversions or at least possible course changes. As people point out in the comments its possible the airlines are diverting the aircraft to airports they can better provide support/maintenance at.