@silverpill collections for GoActivityPub are not really special. A collection is an object which can be operated on with most of the Activity vocabulary to various degrees of success (based on what's in the spec, or based on what felt sensible at the time).
The only way in which they are *special* is that when processing activities the collection ID gets extracted from the actor it belongs to, and gets operated on (for example to add an activity to an outbox or inbox).