New disclosures (Reg. DD, Reg. E, Reg. CC, Privacy) are probably a good idea if the added person is new to your bank, but none of the regulations appears to include such a requirement. They all pretty much allow for one notice per account. But I'd recommend providing a bundle of disclosures if you expect the customer to know what's happening.
Of course, once our mandatory compliance deadline arrives for the Section 326 USA PATRIOT Act rules, you'll have to verify the ID of the added party and add their ID data to your collection of such things, along with whatever ID copy you have to keep.