For some reason when I add a guest to a group they seem unable to access SharePoint. I haven't changed any settings? Why they get access denied screen?
as Akshay Dhavale said if the users are outside your SharePoint, it will not work. if the users are within your domain, then double check with your Super Reader and Supper User Accounts in Web Application level. SharePoint 2013 Access Denied in Site Collections in Web Application