I don't think there is anything in the Activiti engine. The Identity Service is quite simple as the user guide says :
"The IdentityService is pretty simple. It allows the management (creation, update, deletion, querying, …) of groups and users. It is important to understand that Activiti actually doesn’t do any checking on users at runtime. For example, a task could be assigned to any user, but the engine does not verify if that user is known to the system. This is because the Activiti engine can also be used in conjunction with services such as LDAP, Active Directory, etc."
From past experience of building a workflow application, albeit using a different BPM technology, we had to build that into the application. In the process definition you can assign to candidate users, and groups, rather than an individual user, and then a suitably qualified user can claim the task when they are available, or it can be allocated by a supervisor who knows who is available. Issues can arise if a user doesn't complete an allocated task and then goes 'unavailable'. If that can happen and create an issue then you need some way of an admin being able to release and/or reallocate it as well.