An SO member is a member of the Sea Organization, someone who has signed and taken up his/her SO contract, and is actively working in the SO. This includes new recruits in the EPF; people expediting or on post in service orgs and management orgs; people out in the field on tours or missions; people off-post doing MEST work; people on the RPF; people who have been trying to leave for days or months or years.
Every SO member has to be approved by a Fitness Board. This is often a rubber-stamp affair. When an SO member is tossed out, he will receive a Fitness Board. The Fitness Board will find him unfit, for one reason or another. The phrase, "Fitness Boarded out" is often used in such cases, as in "Sally got FB'd out to handle her debts. She should never have been recruited." People who blow are sometimes given a Fitness Board after the fact, and it will find them "unfit" to be in the SO and specify a Return Program such as 1) Pay off freeloader bill 2) Train to Class IV 3) Get through OT III 4) Rejoin via the RPF.
There is a gray area. Some celebrities have had the status of "Honorary SO Member", which roughly means that they would like to embrace the goals of the SO and dedicate their lives to the cause, but it would not be in the best interests of the CofS to have the celebrity doing what regular SO members do. So they do their usual thing in the spotlight and make the name Scientology more well-known. Like Tom Cruise is doing.
Another part of the gray area involves SO members who go on a leave of absence for a shorter or longer period. An SO member on a three-week annual leave is still an SO member. One who is out on an approved LOA to handle a chronic medical condition may consider himself to be an SO member still but probably wouldn't be by other SO members, although he may not have received official notification of that fact.