Identifing clients from a different client

I want to identify a client from another client. Then I want to send the 'name' or ID of the selected client to the server to run a command on the client. I am familiar with server and client side commands, but I do not know how to id the client, from another client.

To be more clear, here is an example of what I am talking about :

User 1 clicks on User 2 and clicks a button that says : "PM the selected user". This performs a call to the server that contacts User 2.

Thanks.