Users with accounts.

(I should also specify, since mark reminded me -- someone involved needs to have a paid account: either the journal being searched, or the account doing the searching. So, if your account is paid, and you have "any logged-in user can search" set, anybody can search it; if the journal you're looking at isn't paid, but your account is, you can search it; if both you and the journal you're looking at aren't paid, nobody can search it.)