Maybe it depends. What if some people are in the group for the satire (and assume the whole group is a joke), and others are in the group because they genuinely believe the earth is flat (and believe they are in the company of many others who believe the same)?
As for 'how do they believe it', I recommend the Vsauce video 'is earth actually flat?'. It's 10 minutes long, and well worth a watch.
So yeah I mentioned him before my cousin who swears to god the earth is flat. He thinks other planets are either flat round or otger shapes. He thinks "it is stupid to think the planets come in only one shape"
As for 'how do they believe it', I recommend the Vsauce video 'is earth actually flat?'. It's 10 minutes long, and well worth a watch.