SCIENTOLOGY® and DIANETICS® are trademarks and service marks owned by the Religious Technology Center, a senior CofS organization. The CofS has trademark information here  but more complete trademark information is available in Scienowiki at trademarks and pages linked from there.
"SCIENTOLOGIST is a collective membership mark designating members of the affiliated churches and missions of Scientology" is an exact quote from that CofS page. This is an example of the CofS applying the PR technique described by Hubbard as Propaganda by redefinition of words. In the 1987 reprint of the Technical Dictionary is the term SCIENTOLOGIST™, and the first definition, by Hubbard, is "one who betters the conditions of himself and the conditions of others by using Scn (Scientology) technology." It would appear that what the CofS is trying to do is to narrow the definition of Scientologist to exclude people using Scientology tech, as developed by Hubbard, outside the CofS.
Also note that SCIENTOLOGIST was not filed with the US Patent and Trademark Office until April 2006.