What you want to do is something like this
with objBoos
{
//Calculate the boo's current distance from the center (or you can just use a constant)
var dist = point_distance(other.x, other.y, x, y);
//Calculate the boo's current angle from the center
var dir = point_direction(other.x, other.y, x, y);
//Add the number of degrees of the circle you want the boos to move each step
dir += other.RotSpeed;
//Use trig to calculate the boo's new x and y coordinates given the new angle and using the same distance
x = other.x + cos(degtorad(dir)) * dist;
y = other.y - sin(degtorad(dir)) * dist;
}
For best results the sprite origins should be in the center of the boos