BCG uses a mailing list hosted at groups.io to communicate with members. When you were admitted to BCG, you were also invited to join this mailing list/group. There are some additional resources available at the group server if you log in:

• access to an archive of past messages 
• auxiliary files (librettos, the roster, etc.)
• ability to change some administrative details, such as the email address you use for the group

When you first join the group via the web, you are temporarily logged in. You can take this opportunity to use any of the aforementioned resources. Once your temporary login expires, you can regain access by visiting the login page and clicking on the button labeled "email me a link to login". The server will mail you a special link, which, when you open it in a browser, again gives you temporary access.

Managing your groups membership
There are many settings available to you, but here are the most interesting. To do any of the following, get yourself logged in as described above, and then click on your name in the upper right corner of any groups.io page and choose "Account".

Change your email address
To change the email address you're subscribed with, use the first section. Replace your email address in the first field ("Email") and click "Change Email".
Email aliases
The group is set to only allow email to be sent to the group from a member. If you normally post from a different email address than the one you are subscribed with, you can add it as an "alias". Click on "Advanced Settings For Email Aliases" at the bottom of the account page. Fill in the address you normally send from, and click "Add Email Alias". This can be helpful for people who use a forwarding service to maintain a "permanent" email address, as is offered by many alumni and professional associations.
Permanent login
You can create an official login for your membership, which will allow you to login directly at a later date without going thru the extra step of asking for a login link.  You have a choice of two methods:

• Fill a password of your choosing into the Password field (please choose one you're not using elsewhere with this email), and click "Set Password". Later, you'll be able to login with your email address and this password.
• Turn on one (or both) of the checkboxes in the "Social Logins" section. You have to already be using  an email address for which you have a Google or Facebook login. This allows you to login by simply clicking the "Login with Google" or "Login with Facebook" button on the login page.

In either case, be sure you click the corresponding "Set..." button, or your changes will not take effect.

(Most people will probably find it simplest to skip all that and use the indirect method of getting a login link mailed to them on the infrequent occasions they need access.)

By the way, if you already created a login for yourself for some other group on groups.io, you don't have to do so again—that login knows about all the groups you are a member of with the same email address.