so you basically want a state-machine ?
There are several approaches to your requirement I guess:
do you want to retrieve the specific event response ? if yes there has to a be an ID that can be used to fetch the specific response. only 1 time or does it need to be persisted ?
if non of the above is true the answer is "a queue". because that's basically "1 user does something -> it ends in the queue -> another user picks it up.
if it's about broadcasting or pubsub patterns you should definitely use a Queue. maybe zeroMQ or RabbitMQ or beanstalk there are several out there.
You need to be more specific so we could give you a more specific answer -> what I did was basically just stating the obvious without knowing your constraints or purpose.