If the DelegateTask interface has the getAssignee() method, it should also have the getCandidates() method of TaskEntity, which is the only implementation.
getCandidates() returns a Set of IdentityLink objects. The interface IdentityLink is also part of the public API.
A problem might be that the returned set does not seem to be usable for adding or even removing candidates. Adding candidates is possible through the addCandidate* methods, whereas removing them seems to be unsupported by the current TaskEntity.