You have flexible options for tagging all documents in a family, and can choose what works best for your workflow: using the bulk tagging tool , tagging from the document list or tagging from the document viewer .
1. Tagging using the bulk tagging tool
While viewing the search results, you can Select All > toggle on families > and bulk tag from there:
2. Tagging from your document list
If you're in the search results you can toggle on 'Family' and checkmark the original document, which will checkmark all documents in the family, and you can tag from there:
3. Tagging from the document viewer
If you're in the Document Viewer, under the Tag section in the Document info pane on the right-hand side, you can select the ellipsis (...) next to the tag name, and an option to tag family will come up: