I'm not sure specifically as I haven't read it. But generally, a 'yuusha' might refer to a hero in Japanese. So it could be a web novel about a hero or a group of heroes in a party setting, perhaps on some kind of adventure, facing challenges and foes.
Well, 'yuusha party wo tsuihou light novel' could potentially tell a story full of drama. Since it involves a hero being expelled from a party, there could be a backstory to that expulsion. It might be due to a misunderstanding, or perhaps there are power struggles within the party. The hero then might embark on a journey to prove himself or find a new purpose, with various characters and events along the way.
I'm not sure where you can specifically read 'yuusha party web novel'. You could try searching on popular web novel platforms like Wattpad or Royal Road. Sometimes, less - known web novels can be found on niche reading websites too.
You might be able to find it in some specialized Japanese bookstores if you're in an area with a large Japanese community. They often carry a variety of light novels.
Yes, they are quite popular. The story of Tate no Yuusha has an interesting concept that attracts a lot of readers. It combines elements of fantasy, adventure, and unique character development.