1- Sit previously used employees and ask their opinion and tell them to develop a plan.. For instance... "you guys used it, how can we make others also use it"
2- Tie them up with an incentive goals to signup more employees / group champions to work together
3- Arrange informative training sessions in your company, raise awareness, ask for enrollments
4- If benefit is a third party contractor; Get more "juice" from contractor and tell your internal champions they will have more benefits if they sign up more.

I remember once we wanted to increase the foot traffic to one of our restaurants. We gave 25% discount to our employees to come and try, didn't get much traffic. We than offered a "refer 3 full table / get free dinner up to 4 people" than we increased our bookings and spread the name of the venue. I guess it really depends on your benefit, if it is an internal benefit or an outside contractor benefit that pays commission to your business i.e. insurance policy...