Log-in instructions vary according to the website they are for, and can be found on each website's membership information tab, such as those listed on the Society of Architectural Historians (SAH) and International Society for Computaztional Biology (ISCB) websites. The membership tab, although varied according to the website, generally hosts information in regards to how to become a member of the affiliated site.
SAH displays a page describing how to activate a member account, along with how to create a member or nonmember account. On the bottom page are how-to instructions for logging in, specific to the website.
The ISCB provides log-in instructions from the bottom of the membership page.