Squall is not invisible! All you can see is Rinoa because there is a PILLAR blocking anything on the right of her. And Squall not being at the party is not exactly strange. He is still a loner-type guy, just not as bad. And, why would they be having a PARTY if their leader had just died? And if, for some strange reason, they were in fact having a party, why were all of Squall's friends

a) at the party
b) happy.

Especially RINOA! So, unless Squall's friends all secretly hated him, and Rinoa was only dating him because she was paid good money, it would be highly, massively, extremely, excessively unusual for them to be having a party so soon after he died.