The ungrouped folder within in your contact groups is a house for those contacts who haven't been allocated to a contact group. These contacts can be sent to when completing a mass mailing (to all contacts) but not from the group 'ungrouped'.
You can remove these contacts from your account by selecting the group (until the line goes solid green), scrolling down to the bottom of the page, selecting remove, then the option to remove from your account.
Removing these contacts from your account will not unsubscribe these addresses, so if you decide that you have a home for these contacts they can be re added to your account.
For more information on groups, please visit our 'How to...Groups' user guide.