After searching through the SDK files, it appears there is no way to retrieve all accounts shared to a particular user or team using code. If you have access to the SQL server, you can execute a query against the crm database to achieve the same goal.
select * from principalobjectaccess where principalid = 'guid of the user or team'
Now, you have to be a local admin on the sql box or be in the privuser group, i think. There's no filteredview that has this information and because you are querying the table directly instead of querying a view, this is technically unsupported.
If you are willing to allow the code to run overnight, you can loop through all contacts or all accounts and retrieve the users and teams to whom the records have been shared using the SDK, but if you have a large number of records, that could take a while.
