If an existing group registration field is missing a required field (name or description) then new groups will not be able to add that existing group registration field.

Steps to recreate:

Create a new Group.
'Change Registration Fields' for the newly created Group.
Click 'Add Existing Registration Field' tab on 'Manage Registration Fields for <Group Name>' page.
Select a Registration Group from the list that is missing its description. Currently both the cmip5 license agreements are missing their descriptions.
Click the 'Add' button on the 'Manage Registration Fields for Group <Group Name> page.
A required field error should occur.