When looking at a list of all the official GEDCOM tags (lists such as GEDCOM Tags and GEDCOM 101 - Tags in the GEDCOM 5.5 Standard, one needs to be aware of the applicable GEDCOM version level. This determines whether the tag is still valid and where it is supposed to occur in the GEDCOM's structure. (A nice example of the GEDCOM sub-structure is found at Version 5.5, Chapter 2).
CORP is not a fact or event. It belongs in the HEADER as "Name of the business, corporation, or person that produced or commissioned the product" (GEDCOM Standard, Draft Release 5.5.1, pages 23 & 54) that generated the GEDCOM.
Other than RELI (religion), there don't seem to be any tags that can be used for organizations that the individual belonged to. EDUC (Education) is the "Indicator of a level of education attained," but, in my database, I will include the name of the institution in the description, and I use a custom tag called "Hobbies and Interests" for non-occupational organizations.