How to tell the guest to register?

Hi!  

I want to have the possibility to invite the guest of my forum to register or log in like on the forum Model Ship World. The link to this forum is here. The invitation label is located in the lower right corner of the browser.

To do this, between other things (like HTML, CSS and JavaScript), I should know is there some variable, for this guest, where he/she is marked as unregistered or not logged in? After he/she ended registration or logged in, the state of the variable should change and the invitation label should be closed.

How to reference this variable (what is its name)?

Thank you for the help.

Kind regards,
Hrvoje

maybe homepage message ACP - DISPLAY - HOMEPAGE - GENERALITIES ?

Hello, andiikadriu!
I am, sometimes, blind for obvious. I will check all those variables mentioned there and see if I can use some of them.
Thank you for the answer.

Regards and have a nice day,
Hrvoje