I want to provide members of a social group a universal login to access the website. I just want a single username and password that everyone who knows it can use, but if you aren’t part of the group you won’t know the password (theoritically).
#featurerequest
Hi Kimberly!
Technically you can achieve this by having a password protected page and sharing that password with your group. 
As @sheyla says, password protect or make the whole site members only