how to add all the subscriber to an email with task owner of workitem ?

By Default it goes to all subscribers. Let me clear this point .

1. Once you add anybody as owner he will be added in subscribers section of workitem and will receive all changes being made in workitem
2. Once you mention someone in comment he will be added in subscribers and will receive all changes being made in workitem
3. Once you change owner from 1 person to other both will be added in subscribers and will receive all changes being made in workitem.

Now to enable subscriber emails it depends on every users own preferences. You can set the preferences like when to receive emails. Please find the attached image below.

Note :  You can below screen by --> Open https://server-name:9443/ccm/web then click on Name icon top right of the page then click my Profile and Licenses --> Mail Configurations.