Congressional Member Organizations (CMOs) and Informal Member Groups: Their Purpose and Activities, History, and Formation [Updated January 23, 2019] [open pdf - 1MB]
From the Introduction, "Several hundred informal Member organizations exist within the House of Representatives, Senate, or between both the chambers; these organizations typically reflect Members' shared legislative objectives or representational interests. These groups may commonly be described as congressional caucuses, working groups, or task forces, but in this report, will be identified, collectively, as informal Member organizations, to avoid confusion with official party caucuses. An additional distinction between informal Member organizations may be drawn. In the House of Representatives, some groups may register with the Committee on House Administration to form a Congressional Member Organization (CMO). CMOs registered with the Committee on House Administration can include groups exclusively for House Members or bicameral groups that include House Members and Senators. Informal Member organizations that are not registered with the Committee on House Administration are called informal Member groups; these include groups exclusive to the Senate, which does not have any formal registration process for informal Member organizations. These distinctions are described in greater detail in the sections below. The Appendix provides some considerations for House Members seeking to form a CMO. Some of these considerations that are not exclusive to the House process, such as determining a group's objective and possible membership, may also be of interest to Senators or House Members seeking to form an informal Member group."
CRS Report for Congress, R40683
Congressional Research Service: https://crsreports.congress.gov