Actors . properties > get actor group

In this case, the group is objects and its ID is 5. Will very often differ from game to game. the resource id can be gotten by using an unused behavior in design mode > Preview code. 

code mode requires import com.stencyl.behavior.Script;  

snippet

preview code in design mode

getActorGroup(5)

code without import com.stencyl.behavior.Script.*; 

Script.getActorGroup(5)

computational shortcut level 1

Engine.engine.getGroup(5);

computational shortcut level 2

Engine.engine.groups.get(5)