Hey David — does your contract with the SEO company already have an NDA? A good contractor agreement should, which would then cover access to Analytics.
If not, or if you're still evaluating that SEO team and don't have a contract with them, I'd definitely use an NDA, even a simple boilerplate one. When I worked at Flippa (a marketplace for buying and selling websites) we would always have an NDA in place before potential buyers got access to the analytics for high-traffic or highly competitive sites. Even a boilerplate one with a few details filled in will define expectations for both parties.