Why not create a collaboration entity table and for each company that is in the collaboration create a collaboration entity record?
CollaborationId, Descr
1, Some Collaborative Event
2, Some Other Collaborative Event
ComanyId, CompanyName
1, CompanyA
2, CompanyB
3, CompanyC
CollaborationEvents
CollaborationEventId, CollaborationId, CompanyId
1,1,1
2,1,2
3,1,3
4,2,3
5,2,2
Then select all company entitys that are associated with the Collaboration Id = 1 and you get companies A,B,C back.
If you want to find all companies that Company C has associated with via collaboration you could do this as well because in the data above, company C has collaborated in events 1 and 2. So you could loosely associate at some point C has collaborated with both A and B.
Hope this helps.