This might not be my place given my recent reappearance but having organised a few of these in the past. I would give the following advice
Someone needs to take the initiative and state date and location
Once that is stated it shouldn't be changed
The agenda can be discussed but should be set reasonably soon after the thread starts
Make it over 18s if you are going to be drinking
Meets that are more than one night are good fun but require more notice for people to arrange accommodation and holiday
Meets of more than 3 days need a specified organiser and someone to look after buying food etc (these are only for the advance organiser

) The BBQs were great fun to do but took a lot of organisation and bulk food buying etc.
But the main thing is; set a date and a place and DO NOT change it, there will always be people who will be disappointed and can't come but reorganising to accommodate will result in annoying people who already said they would come and now found out they can't
Good Luck, meets are one of the most satisfying things to do when they work and you get to make solid IRL friends, I met some of my best friends through the original OcUK meets and over the last year have organised meets in France and all over the country
MB