Why So Many Strange USERS Being Added to My Sites?

Hello –

About 3 days we did a malware check with Sucuri Security because we realized our server was sending spam. They identified a number of bits of malware code throughout many of our sites. It was cleaned out successfully. However, we’ve been getting a large number of “new user registrations” recently across all of the sites we host. Particularly strange, is that these new users are even signing up to sites where there is no signup form. Is this something we should be concerned about? We are a bit jumpy since we found the malware. Thanks!