If you add a member without an email address, you can't (at the moment) add an email address to this existing name-only member. It needs a bit of extra checking to make sure their email address is correct, so they can click a verification link.
Instead, you'll have to invite them again, this time with the email address and then email email@example.com to have the name-only user removed. This also applies to any other duplicates you find due to mis-spelling, typos etc. We are happy to action them for you as often as necessary and can usually do this within 24 hours.
Although we do allow name-only users, we would discourage people from adding people without emails unless there is no plan to ever add an email address for that person in future, e.g. somebody who does not have an email, or is a temporary helper. Generally speaking if you do have an email address for somebody, wait until you have it handy rather than just adding their name. Otherwise when they do join with an email, we can't link them up with the tasks you have already assigned to that name-only user.