A GUI should appear when you join with different nick, it let you register a primary account( primary account will allow you to play as guest/unregistered player after you login ) but registration of primary account needs admin approval. If the GUI didnt appear, there must be something wrong in the client-side code. You have to PM me or post your debuglog here, it's located at:
%appdata%/VCMP/04beta/
Make sure you join EC server first, before opening the debuglog because the log is updated everytime you join a server.
More information about the update:-
https://ec.vc-mp.co.uk/index.php?topic=5406.msg42780#msg42780