What exactly do you mean?
1) Does the dynamic action only be fired if a specific is loged id and using the page the dynamic action is located.
This is what the plugin is intented for and a condition will do what you want.
2) Or user A is using the application and triggers the dynamic action and then user B sees a message when using the same application but on a different computer.This involves much more than a dynamic action.
Something much sent a message to the server. On the server a message is generated adressed to user B. On the application a process is listening to the server for messages adresses to the user that is loged in.