1. Open the Designer
2. Click 'Show Hidden' in the left navigation bar.

This will show you a list of all Report Groups that have been hidden on your account.
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If you'd like to reinstate a group back to your main Report Group list, simply hover your cursor over the group name, and click the 'View' icon that pops up:
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This will move that group back into your main Report Group list.
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​Note that any changes made here will only affect your personal account. Everyone on your team can view/hide report groups individually, so these changes will not affect anyone else in your company.
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Hope this helps! Let us know if you have any questions!