re: which value is the unique identifier to use Link
The WordPress user is linked to the Connections entry ID and is stored in the usermeta with the key name of
connections_entry_id with the value being the entry ID.
re: How do I import the contacts within Connections? Do I need to buy another plugin for this?
Actually, there is no way of doing this without writing an extension to do it.
re: And how do I make sure the both addressbooks are linked?
That’s what Link does.
Does that help?