Thanks for your reply, but I don't think my problem has anything to do with the groups.
My problem is that even if I add them to another group, there doesn't seem to be a permission that only gives them 'Ready to Publish' rights, i.e. if they have 'Create' rights they can Save pages but don't have the Publish or Ready to Publish options. If I give them 'Publish' rights then they have Ready to Publish but also Publish which I don't want.
I can't see a way to give them one without the other!