There does not seem to be a way to know if a member has ever logged into the website. Nor a way to know if they have setup a password. Both would be useful to know.
I recognize that many will only lurk reading emails. But if I want to send a poll, responders have to login to answer the poll question - it would be nice to know how many members have ever seen the website.
I can imagine several fields:
Posted/replied via web
Posted/replied via email
Logged into website*
Set account password*
*hmmm - this is account level not group level, but should still be available via an export of a group's member.