Use a join table like this:
class User {
Set<Node> getNodes() {
UserNode.executeQuery("select x.node from UserNode x where x.user = :user", [user: this])
}
def addToNodes(Node node) {
def userNode = new UserNode()
userNode.user = this
userNode.node = node
userNode.save()
}
}
class Node {
Set<User> getUsers() { ... }
}
class UserNode {
User user
Node node
}