I wanted to add a contact to an email list but couldn't, and it took a call (ok, a chat session) with support to figure out that since the contact was marked Unsubscribed, he couldn't be added to any email list.
In Contacts, I searched for the contact. Then I checked the box on the left side. It said "1 selected", but the "Add To Email Lists" button was greyed out.
I suggest that you have a column for Unsubscribed on the selection screen so it is easier to tell why a contact doesn't activate the Add To Email List button.