Interesting. I have a whole lot of comments regarding this but not sure how to say them without a very long statement.
Short response. You should be able to add an individual of any kind (related or not related) into your database. Create a "Multi-Participant Fact/Event" and allow any number of individuals to show a connection to that Fact/Event with a TYPE to designate what function or acts caused them to be included. This DOES NOT violate or even effect GEDCOM so no worries. Any individual added to the database as a "non-family individual" should at the least have a Name displayed for any association they have to a "family member" being reported/displayed. I use the term "family member" to represent a person that can be connected to the primary individual either by birth, foster, adoption or marriage where non-family individual are all other individuals that are not "family members".
Associations in GEDCOM only occur as Individual to Individual, so Individual->Fact->Individual is technically a violation of GEDCOM, but again I personally bend this rule because it is logical to have this relationship.