[mme42]

Well, this is not a popup, but it will add a notification box on the top of the forum and you can add a message to register. This is what I do. Personally, popups annoy me and i wouldn't want to use them.

Admincp > Notices > Add new notice

Enter any HTML that you want into the box. You'd then select "Unregistered/Not Logged in" for the option "User belongs to usergroup." You might also want to select "yes" for "persistent" and "no" for dismissible. Those options will ensure that the box is always there for people that haven't registered or logged in. But, that's up to you. Another note, If you do use this method, you might want to think about removing your "Guest Welcome Message" on your index page so that it's not redundant. You can find the code for this in your "forumhome" template and it's marked with comments. You can just remove that block of code.

You could probably also create a popup by entering the HTML in that box and using a javascript file for the popup. Or, you could probably do the same thing with a javascript file and a template conditional (to specify that it should only execute for guests) in the header or whatever template you wanted to add it to.

Or, there's this modification if you do want an actual popup:
https://vborg.vbsupport.ru/showthrea...ighlight=popup