YouTube unit is racing to lock up its top stars as rival online video services court them aggressively. Facebook and video startup Vessel, among others, have tried to lure YouTube creators. Google is offering some of its top video makers bonuses to sign multiyear deals in which they agree to post content exclusively on YouTube for a time before putting it on a rival service. The bonuses can be tied to how well videos perform, but YouTube is making a wide range of offers to counter rivals, according to people involved in the discussions.