Alfresco Process Services needs user, group, and membership information in its database. The main reason is performance (for example quick user/group searches) and data consistency (for example models are linked to users through foreign keys). In the Process Services logic, this is typically referred to as Identity Management (IDM).
Out of the box, all IDM data is stored directly in the database. So when you create a user or group as a tenant administrator, the data ends up in the database tables.
However, typically, the users/groups of a company are managed in a centralized data store such as LDAP (or Active Directory). Process Services can be configured to connect to such a server and synchronize the IDM data to the database table.
See External Identity Management (LDAP/Active Directory) for more information on how to set this up. The basic idea behind it is that the LDAP server will periodically be polled and the IDM data in the database tables will be synchronized: created, updated or deleted depending on what the LDAP server returns and what currently is in the database tables.
This section describes what is needed to have a similar synchronization of IDM data coming from another source. The com.activiti.service.idm.LdapSyncService responsible for synchronizing IDM data from an LDAP/Active Directory store, uses the same hook points as the ones described below and can thus be seen as an advanced example.