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There are several ways you can accomplish something like that.
One way you can do it is to edit your WordPress forum page and add something to it to be displayed to non logged in members using php and echo.

Anything you add to the page will be shown before the forum loads.

Depending on what you allow guests to see and do you can also add in other things to bring attention to registering.

As an example on one of my sites I don't allow guests to post topics but they can view them. So under every last post I put in a line that says they must register to comment on that post.

This is one way you can do it.

Create an image that you want to use for your badge and name it something like admin_badge.

Then go to the Simple:Press forum admin page Components - Forum Ranks. You will see an upload button there that says "Select rank badge to upload:"  Upload your badge image there.

Then select then under Special Forum Ranks enter a rank name like 'admin' and click on "Add Special rank"

Once added you would then select the badge, using the drop down, and selecting the name of the admin badge file you just uploaded.

The click on "Add" and add your admin member/username there.