My suggestion is that the new membership forms are tracked, and then the "New Member Welcome" pop-up(?) message disabled when the form is submitted. Since it seems redundant to suggest people submit it once they already have done so, unless of course you would like duplicates, but I have not gotten that impression. Edit: It might be a moot suggestion, since after posting the pop-up(?) message appears to have stopped popping-up(?)
Actually I think we might be having some session issues on the website, because I get the same thing, and I can't get it to "remember me" either. I think that's probably why it keeps thinking you're new.
The new membership form popup notification ceases to appear once a user has made 5+ posts. This way someone who comes in to post, still has the opportunity to see the membership popup notification, at least for 4 more posts. Just trying to increase the chances of someone seeing the popup
