You share an entity by adding it to a global partition. This can be done from both the sharing host or the sharing guest. You can also create a new entity directly in a global partition.
A global entity is an entity that is shared across multiple independent Security Center systems by virtue of its membership to a global partition. Only cardholders, cardholder groups, credentials, and badge templates are eligible for sharing.
To share an entity with another site:
On the sharing host, the effect of this action is immediately visible. On a sharing guest, the newly shared entity does not appear until after a synchronization is performed on their GSC role.