To spend the steps in your bank to your group walk you must first join the walk which has been started for your group.
To do this via the website, click on My Groups where you will see your group walk is available to join. Click on it and then on the Join Now button.
To join the group walk using our app, follow the advice in this Help Section article: https://worldwalking.org/help/article/5759/how-do-i-use-the-world-walking-ios-app (Our Android app works along similar lines).