As mentioned in https://answers.atlassian.com/questions/261017 in version 3 it was OK th select a group to add reviewers – but in the new version 4.0.3 groups are missed in the possible reviewers list. Permissions checked, so is this a bug?
It seems that you are experiencing the problem reported as part of https://jira.atlassian.com/browse/CRUC-7676.
Please watch the bug report, so that you'll be notified as soon as there is an update.
Caterina - Atlassian Support