I was under the impression that its more BNF than EBNF ... whatever that means :)

meaning you don't use * to specify OPTIONAL rule, or zero or more times ... or whatever you mean by it :) see http://blogs.perl.org/users/jeffrey_kegler/2013/01/making-dsls-even-simpler.html#comment-311460 and http://stackoverflow.com/questions/18634749/marpa-parser-cant-seem-to-cope-with-optional-first-symbol

With that in mind I come up with something which doesn't croak on the SLIF; I've no idea what it does :)

sub get_grammar {
    return q{
    :start              ::= Alias
    :discard            ~ ws

    Alias               ::= 'User_Alias'  User_Alias  repeatUser_Alias
                        |   'User_Alias'  User_Alias
    

    repeatUser_Alias    ::= manyColonUser_Alias
### repeated OPTIONALLY
    repeatUser_Alias    ::= 

### repeated many times
    manyColonUser_Alias ::= colonUser_Alias*
    colonUser_Alias     ::= ':' User_Alias
    User_Alias          ::= NAME '=' User_List
    User_List           ::= User
                        | User ',' User_List

    User                ::= <user name>
    ws                  ~ [\s]+
    NAME                ~ <name_firstchar><name_restchars>
    name_firstchar      ~ [A-Z]
    name_restchars      ~ [A-Z0-9_]*
    <user name>         ~ <username_firstchar><username_restchars>
    username_firstchar  ~ [a-z]
    username_restchars  ~ [a-z0-9_]*
};
[download]

I'm eager to learn what you learn about this :)