Hub user email documentation (#2231)

* docs: add tip about searching users by email in resource groups

Organizations can now add users to resource groups by searching
for their organization-specific email address (e.g., user@company.com).
This is helpful for orgs that identify members by email rather than username.

Co-authored-by: Eliott C. <coyotte508@users.noreply.github.com>

* Apply suggestion from @coyotte508

* clarify email domain

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Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Eliott C. <coyotte508@users.noreply.github.com>

Author: coyotte508

docs/hub/security-resource-groups.md

@@ -42,6 +42,9 @@ After creating a resource group and giving it a meaningful name, you can start a
     <img class="hidden dark:block" src="https://huggingface.co/datasets/huggingface/documentation-images/resolve/main/hub/org-resource-groups-manage-empty-page-dark.png"/>
 </div>
 
+> [!TIP]
+> When adding users to a Resource Group, you can search by email address if the user has an organization-specific email (e.g., `user@your-company.com`) matching your organization email domain.
+
 Remember that a repository can be part of only one Resource Group. You'll be warned when trying to add a repository that already belongs to another Resource Group.
 
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