How to get List of Clans/Groups available to Join?

PlayFab has not provided this feature yet. However, CreateGroup is an Entity API, when it be called on CloudScript without assigning an account entity, title entity will be assigned and title will be the default Administrator of the generated Group. Then Entity API ListMembership can list all groups and roles for an entity. Hence, if all your groups are created by calling CreateGroup with title entity on CloudScript and you keep the title as the administrator of all groups, you can list these groups via ListMembership. You can try to execute the following CloudScript functions via client API ExecuteEntityCloudScript. The above content can be a possible workaround in certain situations. Besides, for any request of new features, you can navigate to Feature Requests, search for the corresponding posts to vote them or add a new feature request about listing all groups available to join. You can also track the progress of new features through the posts of feature requests.

handlers.CreateTitleGroup = function (args, context) {
    var createTitleGroup = entity.CreateGroup
        ({
            // assign the groupname via args here
            GroupName: args.groupname
        });
    return { createTitleGroup: createTitleGroup }
}

handlers.ListTitleMembership = function (args, context) {
    var listTitleMembership = entity.ListMembership
        ({
            Entity: {
                Id: "[YourTitleId]",
                Type: "title",
                TypeString: "title"
            }
        });
    return { listTitleMembership: listTitleMembership }
}