The book was about kids who did a role playing game every summer. They are separated for a summer and somehow fall into the game/world they created. The first one falls into the world through a bush coming back from the library. The world is a bit different from how they originally created it. It has a pink castle and a well, and they know that the bad guy is an uncle who they created. They use rings to meet there at certain times every day to play and live out the world's story.
It sounds very similar to the Space Demons trilogy by Gillian Rubinstein, although it's years since I read it so I'm not sure about most of the details.
The main four characters in the book are Andrew Hayford, Ben Challis, Elaine Taylor and Mario Ferrone. The plot starts when Andrew's dad brings him an exciting prototype video game from Japan. Andrew, who is a video game enthusiast, shows it to his best friend Ben Challis, who agrees to play the game with him. Later, two other players are introduced to the game: Mario Ferrone and Elaine Taylor. It is later revealed that it is possible to get transported into the game by means of a special gun, which only works when a strong beam of hate is directed at someone. Later on, the four get trapped inside the game and gradually work out the only way to escape and thus win the game is if they conquer their hate.
Summary from Goodreads:
Five young people addicted to a fantasy role-playing game are transformed into characters with remarkable powers and sent into a strange and dangerous parallel world.
However, my attempts to Google the inside of the book haven't come up with any significant mention of rings or bushes that match what you've described.