When an event join is trigerred , you still dont have an entity for that client...maby thats the problem smile
You should set that ID number in the player function , so that you know for shure it is attached to a skill of your player entity.

As for the strings , you need a pointer , I think.
Code:
str_cpy(send_str,"Registered");
send_string_to(me,send_str);


Extensive Multiplayer tutorial:
http://mesetts.com/index.php?page=201