controller 

def index
  @groupNotes = Group.getNotes(@current_user)  # this gets all the notes that are shared to this group
  @friendNotes = User.getNotes(@current_user)  # this gets all the notes that are shared to this user

  #i'd like to do a:
  @notesIcanView = @groupNotes + @friendNotes  
  @notesIcanView.sort # ya I know this doesn't exist

model 

In both models I wrote a getNotes function that gets all the notes that the passed in user is allowed to view.