Just to explain Deega's post in more detail...
By default, when you specify a userlevel number it will trigger the event for everybody on that level or lower, so, for example, on 3:join: would trigger for levels 3, 2 and 1. on 5:join: would trigger for 5, 4, 3, 2 and 1.
The + prefix makes it ONLY match the specified userlevel.
Somewhat related is the fact that using on 1:join: will only match people who either aren't in your user list or have a numbered user level; it won't match somebody with a named user level. To match *everyone* you should use *, like so: on *:join:.
