I need to know what conventional practice is regarding "collectors" of collections.

Who would this typically be, if it is not the organization who owns the archived materials?

It all depends on the items in that collection. Collector would apply if an individual or an institution physically collected and/or donated the material (whether it's digital or not). You can also leave it blank if it doesn't apply. We offer many fields to appeal to a broad range of potential Omeka users.