All Gods have the right to attend this meeting, and it is considered unworthy to attempt to prevent a God from attending. (There are exceptions to this: it is possible to become no longer welcome at all in divine society, for example by sabotaging one's Duty or murdering another God). There are also two ways for non-divine entities to gain admission:
Furthermore: The announcement phase begins when the host invites those folk who wish to make an announcement to stand in a line facing the gathered company. By tradition there can be up to six folk making at the beginning of the meeting, and six at the end. It is the right of the host to decide who will be able to make their announcements. You must declare the basic gist of what you are going to announce to the host before the announcement phase (ie, before time-in for the start-of-session phase, during the session for the end-of-session phase), and can't tack on extra announcements in response to other announcements. (You can get away with saying "I'm going to make an oath, issue a challenge, and declare war" without going into the specifics).
If you say "Erm, can I make a quick announcement?" at the end of the phase, when you were not one of the folk who lined up at the beginning of the phase or when you have already made an announcement, the GMs will shout "NO" at you in unison.
Why the rules surrounding announcements? Well, firstly, the Gods (and the alpha GM) know through bitter experience how easily announcement phases can become bloated, unweildly, undiginfied things which take up far, far, *far* too long. They have no intent of allowing it to happen on their watch.
Secondly, part of the Duty of the Messenger of the Gods is to tell mortalkind about the announcements made in the announcement phases. You aren't just addressing the meeting when you stand up and make an announcement: you're addressing the whole of Creation. The limited number of slots reflects their value.
The advantage of hosting the meeting, aside from receiving lots of lovely presents from non-divine guests, is prestige: if you throw a good party, have interesting entertainment and nice food, and the place you throw the party in is an impressive bit of architecture in good repair, all these things can increase your stature and that of your Court. The downside is the need for preparation: finding decent entertainment and food is a Mundane action, and the residences of the Old Gods are all ruined (and so really ought to be prepared before the Old Gods can entertain once more). If you end up throwing a dull party, your stature (and that of your Court) may fall.