The ability to create user groups instead of resource groups to allow restrictions to groups of users like (contractors or contingent workers) that differ from the permissions in the resource group settings. This would be very useful to restrict the number of desk bookings or days on which they can book desk in the office. For instance if you wanted to restrict them to only book desks on Mondays/Friday or a certain number of days. Instead of having to set this for each individual user it would be great to have a user group and apply the permissions to the entire group.