Hi @PrashanthB 
and welcome to our forums!
Are you using Tasklist? If so, the candidateGroups value of the user task is not yet used to block other users from viewing and claiming tasks. AFAIK, Tasklist does not yet have knowledge of user roles. At this time, Tasklist only auto claims tasks for users based on the assignee. See User Task Assignments for more details.
If you use the Tasklist GraphQL API to build your own Tasklist UI, you can access the candidateGroups value of a user task (see task object).