Kann Man sagt "Guten Tag zu Alles" to mean the he is saying good day to all in a group.
I find the usual way to be just
while looking at everybody in the group.
If you want to emphasize that you really greet everybody, you can say
Which I (North German) find more common to say than
Hallo alle miteinander
which feels slightly outdated to me, but still works.
Of course you can replace Hallo with Guten Tag.
The Nominativ case, as suggested by πάντα ῥεῖ and infinitezero, is one (natural) option.
However, there are others. It's possible to use Dativ (whom do you wish a good day?):
Guten Tag Euch allen!
(...) allen Anwesenden einen Guten Tag.
If you use the right preposition, you may even use Akkusativ, e.g.:
Guten Tag an alle!