I wasn't able to find the credit union's field of membership page, however, in their online application, they list several options you can choose to be eligible to join. In addition to being eligible through one of several Select Employee Groups (including the Federal Government), the application allows you to select the option to join a sponsoring member organization. Membership into the credit union requires a minimum deposit of $25 into the Prime Share savings account.
Branches are located in the Massachusetts cities of Boston, Bedford, Burlington, Concord, and in the Hanscom Air Force Base. The credit union is federally insured by the NCUA (Charter # 9095). It has $561.3 million in assets and 42,468 members.