If you are running DEVONthink 3 Pro or Server, hold the Shift key and select the Data menu with more than one file selected.
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Doh! It’s the Data > See also & Classify I was looking for which has now moved to Tools > Inspectors > See Also & Classify.
I couldn’t instantly find it in the menus, found and read this thread and was rather worried the functionality had disappeared completely. Much relieved to have found this !!
No worries. Glad your worries were quelled. 
Selected ~300 inbox items, holding shift in data menu and activated “Group Similar Items”…
Nothing happens after 10 minutes. When I deselect and then reselect another set of files, the option is grayed out and contextual menus are no longer available when right clicking over a file name until I restart DevonThink Pro. Am I misusing this feature?
What kind of files did you actually select?
390 items
706.8 MB
PDF+Text (290), PDF (82), TIFF, MPEG-4, MSFT Doc, JPEG, CSV, .xlsx,
Perhaps exclude items with no textual content (PDF, TIFF, MPEG-4, JPEG) from the classification?
Welcome @JazzyG
DEVONthink isn’t going to magically file all those things in one selection.
Try the PDF+Text documents to start.
PDFs don’t have a text layer, so on what basis would they be grouped?
It’s also not going to group things like images, etc. That likely could be handled to some degree with a smart rule.
Interesting…
Magic was what I was looking for in the absence of documentation to suggest these limitations. Image files have structured data elements that might be detectable and aggregatable. Even the title could suggest a grouping. Considerations for the future…
I am converting the pdfs with OCR and then will test on text-based files
See Help > Tutorials > Understanding the AI for a quick overview on the topic. It also points to the built-in documentation as well.
Thank you for the help.
After applying OCR to the PDF files, I was able to successfully group similar items. I read the documentation and reviewed the tutorial, which are also helpful, but do not state the text-based limitations of the feature. Because DevonThink’s AI suggests groupings for images using the classify feature, one might assume this capability would also be represented in the “Group Similar Items” feature.
You’re welcome and thanks for the feedback.
Images are actually supported.