How to share a collection of sbol designs in SynBioHub
There is a balancing act between allowing all edits, which defeats the purpose of having URIs point to static objects, and allowing no edits, which defeats the purpose of providing some updated information about an object.  SynBioHub resolves this issue by allowing a limited amount of edits after publication.  To answer your questions specifically:

Yes, you can share objects by generating a share link OR adding another owner to an object.  Sharing can be done by the original owner of the object.
There are some mutable fields that any owner of the object can change, these include the description, notes, source, and references in the Details section.  You can also add attachments.
Yes, when you publish (Make Public) a new object or collection, you have the option of making it public into an existing collection OR a new collection.  A person with curator privileges on the SynBioHub instance you are working on can perform this operation.
Answered above.