Every jammer will need to be registered on the GGJ website by the Saturday of the event. It is recommended you and your team go round and talk to every jammer in attendance, and make sure they can register and add themselves to a game project.
If you don't need to take payment, or ask for any extra information on jammer sign up, it is recommended that you use the GGJ registration system for your jam.
Some organisers find registering their jammers on a external site more convenient, in terms of needing features like allowing jammers to pay for tickets.
You should note however that you will need to register you jammers on the GGJ website by the Saturday of the event.
There are lots of 3rd party ticketing systems that you could use, and most a free for free events, or will charge a small % of paid tickets.
Some external systems you could use:
Eventbrite - one of the most popular ticketing websites
Ti.to - an alternative to the above with a big discount for community events
Jamhost - a ticketing site designed specifically for jams, and maintained by folks in the GGJ community
In the next couple of years, GGJ hopes to update it's registration system that will allow SOs to charge for tickets, and add custom questions of jammers that sign up, to streamline the registration process.
We do our best to try and track how many jammers attend GGJ each year. For this reason we also need you to remove those who are not in attendance at the jam. Someone at GGJ may contact you if there is a high number of people at your site who are not associated with a game project, to check that this happened.
You can also add and remove jammers to your site yourself (although they need to be registered on the GGJ website already). You can also download all of your jammer info, so you can contact them in advance with information about the jam.