I'd like to be able to display the number of "active" members on a site. Ideally this would be something like "number of members who have logged in in the past 30 days".
Barring that, I'd settle for the total number of combined members in a set of member groups, like: group_1 + group_3 + group_5 = total number of members.
What do you think is the best way of going about this?