If you are currently a member of the society then a username and initial password will be e-mailed to you when it has been created. You don't need to do anything.
If you are not currently a member, then you will need to communicate your desire to be a member to the society. The current contact mechanism is to send an e-mail to one (or more) of the persons who can authorize you as a member. The primary criteria for membership are expressing an interest in the Sherlock Holmes stories and related things (pastiches, movies, etc.), and participating in the meetings of the society.
The communication mechanism will be set up shortly; stay tuned!