Steps to reproduce:
1. Set up Liferay to be able to send emails.
2. Start the Portal and deploy Kaleo Workflow 1.0.2 EE.
3. Create a User Group.
4. Assign your User Group to the default Liferay Site.
5. Assign the Site Content Reviewer Role to your User Group.
6. Create a user and assign him to your User Group.
7. Assign yourself to the User Group.
8. Set the Single Approver workflow for Web Contents.
9. Create a Web Content, submit it for review.
10. Check the My Workflow Tasks tab, and try to assign someone else.
Result: you won't see the "Assign to..." option, furthermore you didn't get an email about the Web Content submitting.
Same results on 6.1.x with latest Kaleo.