Remind them of basics like the venue location, check-in time and transportation. It is also recommended give them some guides on a few good-to-have preparations: such as sleeping bags, toiletries, re-checking their jamming equipments and more. Consider sending a guide to your jammers so they have all the information in one place. Example guides:
Melbourne GGJ site, Australia 2019 guide
If you aren't already using a community platform, consider using a Slack or Discord channel for venue announcements and comms for your venue. But really anything works, depending on what most of your jammers are used to, such as Facebook pages & groups, Whatsapp, WeChat etc.
GGJ now has a dedicated Discord server for all jammers to participate in! Your jammers can get in touch with participants from across the globe, asking game dev questions, making friends and promoting their own GGJ games to others. There are also regional channels setup for local comms. All in all, a great way for your jammers to feel connected to the global community.