Assuming you have previously
you can now check when the next ceremony will take place. If you would like to attend the upcoming ceremony, you can tap the register button. You will need a tiny bit of inital funding to pay transaction fees. Either you get community currency from someone in your community, or you get KSM, the native token of the Encointer mainnet on Kusama.
According to the ceremony cycle you will have to await the assigning phase until the app shows you if you have been assigned and when you have to be in which location.
On the day of the meetup, be on time at the right location and perform a ceremony meetup with your fellow attendees you will hopefully encounter there.