Originally Posted by Catgirls
It appears to be written both ways and yes, http://kasimasi.com/
is the official site. If you notice between the two spellings the only letter missing is the "h" (twice):
Kashimashi ~ Girl Meets Girl ~
Kasimasi ~ Girl Meets Girl ~
Maybe it's a matter if dialectic or pronunciation.
Both "s" and "sh" are accepted romanizations of the hirigana because it is pronounced differently in different dialects.