In each record type and in several areas of Mercury, you will find the Share icon.
It is in the ribbon on the top bar of most menus and records.
It is not common that this will be used within Mercury, as records are typically accessible by all users.
It is mainly used where a user wants to share a record that another user cannot access (where security prevents them seeing it normally). Most of our clients don't use any business unit security but this would be an example. User in Business Unit A wants to share a Vacancy with someone in Business Unit B. They use share here to make it available.
The one record where it is particularly useful is sharing hotlists to other team members or to your whole team.