Rabbinical graduates from JTS, the University of Judaism (California) and The Schechter Institute of Jewish Studies (Jerusalem, Israel) are automatically members of the RA; rabbis whose ordination is from other seminaries and Yeshivas are also welcome to apply.
The Committee on Jewish Law and Standards (CJLS) is the movement's central body on interpreting Jewish law and custom; it was founded by the Rabbinical Assembly in the 1920s. It is composed of 25 rabbis, who are voting members, and five laypeople, who do not vote but participate fully in deliberations. When any six (or more) members vote in favor of a position, that position becomes an official position of the Rabbinical Assembly. When more than one position is validated, each congregational rabbi functions as the mara de-atra (local authority in Jewish law), adopting for their congregation the position he or she considers most compelling.
The Rabbinical Assembly of Israel (Israeli arm of the RA) has its own decision making body, the Va'ad Halacha. Responsa by both the CJLS and the Va'ad Halacha are equally valid, although the Va'ad's emphasis is on issues pertaining to Israeli society. The CJLS and the Va'ad do not always come up with the same answer to a question. In such cases a rabbi is free to decide which responsa to adopt.
There is a separate article which has details on Conservative responsa.