The last round of international conventions were in 2006? (maybe 2007, announced in 2005).
Anyway, the letters to the congregations began with elders who had been serving 10 years or more could apply.
The next letter asked for any elder who had been serving for 5 years as well as pioneers.
The next letter asked for any elder and pioneer.
Now, they've decided that instead of embarrassing themselves by slowly lowering the requirements they would just open it up to any approved publishers.
I distinctly remember my wife and I snickering how funny it seemed that for a several month period letter after letter was read which lowered the attendance requirements. (remember, snickering is a big deal for her because she's a dyed in the wool dub).
Note: anyone can attend the conventions, they are open to the public, however, if you are a foreign delegate you won't be able to travel with the approved dub groups. Also, if you go to just any old international convention you would need to ensure your language will be supported. Even with conventions in the US they don't want dubs to randomly travel to any old DC they choose. The WTS has stated several times that dubs should notify their congregation secretary that they will not be attending the DC they are assigned to and which one they plan on attending. For food and room requirements plus ensuring enough seats are available for all planned to attend a particular convention (BS BS BS BS BS BS).