If he's Army, AR 600-20 says no. He can be there as a participant (but not as an official) as long as he is not wearing the uniform. There is a long list of rules restricting the political activities of our servicemembers.
Depends if it is a gathering of bloggers talking about blogging or a political event.
Sounds like they think it is a political event. So maybe we sould be looking at YearlyKOS violating campaign laws.