OK, in that case the JOIN message you receive from others will look something like this:
:joininguser JOIN :#joinedchannelname
The exact format of
joininguser varies depending on the server, but generally it will be a full
nickname!ident@hostname.com. The colon (
:) before the channel name is also optional. This is briefly described in the "Join message" section of RFC 1459 (section 4.2.1) however it's not very well explained.
As a note, you can use mIRC's debugging feature to see the exact raw messages it sends to and receives from the server. For example type
/debug @debug and the
@debug custom window will list all traffic.