Members online is some information that doesn't look like
it depends on the request being done. Often, for web pages,
"members online" isn't anymore more than "this many members
accessed the site in the last X minutes". If X is say, 2 minutes, and your site is very active, you might consider
updating your index page every 2 minutes (if your site isn't
very active, updating every 2 minutes doesn't hurt).
Unless it's about pages whose content isn't known until the
request is made, I wouldn't dismiss static pages to quickly.
Abigail