How to Identify and Mark Leads from the Same Company/Domain in Apollo

So I a bunch of leads from Apollo and I think some are from the same company/domain. I'd love to mark all the companies from the same domain (maybe using a formula?) so that I can then review all of those and figure out which contact I'd rather reach out to. Is there any way to do this? When I use the "de-dupe" feature, it shows me 300 duplicate company's, but doesn't give me the option to mark them so I can review.