I was working with this module and found I was unable to filter my view of sign-ups to show who had not attended. Quick look at the database revealed that records had "NULL" entered in the attended field instead of 0. 0 is what the views filter is looking for.

I modified the signup_log table to not allow NULL and use 0 as the default value. I also modified signup.install around lines 129-130 to add in the not null and default value attributes. From my limited testing, this seems to resolve the issue.

As I typed this out, it occurred to me that NULL may be used as a way to separate out users who haven't been marked as attended or not attended. Is this the case? If so, a new views filter, or addition to the existing filter, would be helpful for building a list of users who haven't been marked "attended".