When editing the details of a review and trying to add a group, if the group has uppercase letters in its name it is not possible to filter by this group name.

The group still shows up in the list, but if there are many groups, due to the size of the list it is not possible to add this group to the review.

Workaround 1
Add the reviewers manually one by one to the review, searching for their name.

Workaround 2
In Administration > Projects edit the project in question and, under Default reviewers set the users and groups that will be added to the reviews by default when they are created.

Workaround 3
Use the reviewers function of the Rest API to add reviewers to a review.