When Is A Business Entity Formally Recognized For Plan Purposes?

An employer that is fully owned by one individual sponsors a qualified plan in which several employees participate.  This individual has also set up a separate shell LLC (in which she also has 100% ownership) that does not perform any business transactions, and for which no business tax returns are ever filed.   She does not receive any income from the shell company, and it does not have any other employees. 

I'm trying to determine whether I should indicate that the plan sponsor is a member of a controlled group on the 5500-SF.  Is just signing a document establishing the shell company the only thing needed for it to be considered a controlled group member?  Or would it also need to first have an EIN issued?  

It is a controlled group whether they sign the document or not.  The reason for adopting the document is to be a participating employer and has no bearing on the controlled group status.  

RatherBeGolfing is usually right, and he may be right here, but I would point out that under the "check the box" regulations a single member LLC that does not elect to be taxed as a corporation is "disregarded" for almost all federal income tax purposes.

Im not sure what planet I was on when I read the OP because I missed all the relevant information...  For some reason I read the "sign a document" portion of the OP as singing the plan document.  Please disregard my prior comment.

@Luke Bailey good catch on the disregarded entity.  I guess the question then is, can a disregarded entity be part of a CG?  Sole props are, but what about single member LLCs?  Is there a simple answer?