User Group needed to be add to Downloads >> Categories

[Armen]

Hi guys,
We need to bind the Downloads>>Category to the User Group, so that when creating a new Category the "User Group" field must be added to the appropriate form, where any of 'U', 'UQ' or 'UR' values must be specified. Hence the Category<->User Group relation is established, when the user logs in the Downloads area, he can access the particular Category only if he has the appropriate User Groups level. For example, if for the particular Category the User Groups has been set to "UR", only the users belonging to the 'UR' group may access to that category. For the 'UQ' and 'U' users the category will be invisible/closed.
Does anybody know how can we do that? Any kind of algorithm? I will appreciate any suggestions or ideas?

Thanks in advance,
Armen