Ability to sort contacts into groups

In addition to individual Twitterers, the user should be able to select/deselect groups with all the same options. Groups will contain a subset of followed Twitterers, and a Twitterer can be tagged for more than one group. A group can be tagged for another group. For example: I would assign person A to group "Linux", but not to group "Ubuntu", persons B and C would be "Ubuntu", person D be in neither group. I then tag group "Ubuntu" for group "Linux". If I select only Ubuntu, I would get tweets for B & C, If I select only Linux, I get A, B, and C, because Linux contains Ubuntu.
If multiple groups are selected, individuals will not be duplicated. (selecting Linux and Ubuntu in the above case would not result in double-tweets for B & C)
Some difference must be maintained between groups and people so that a group named "Ubuntu" could not be confused with a possible Twitterer named "Ubuntu".