I don't think it is possible to share a database link between more than one user but not all. They are either private (for one user only) or public (for all users).
A good way around this is to create a view in SCHEMA_B that exposes the table you want to access through the database link. This will also give you good control over who is allowed to select from the database link, as you can control the access to the view.
Do like this:
create database link db_link... as before;
create view mytable_view as select * from mytable@db_link;
grant select on mytable_view to myuser;