Hmm, I'm no expert on .htaccess redirects. One thing you can do is write a script("members"), use an Apache ForceType directive to parse it with PHP, and have the script read the username from the URI request string ... dunno if this would be what you want, but it looks close.
For example, I have an "articles" script. It reads the article name thus:
http://mysite.com/articles/Read_this_please
and the script pulls the article with that title into the page.