Once you have created user groups, you can give them permissions over folders as described in Folder permissions.
Folder permissions apply down the folder hierarchy, so giving a user group a folder permission gives members of the user group permissions over the folder, all its sub folders, and all the tasks in the folder and its sub folders.
If you apply a permission after adding a sub folder, you will need to refresh the folders to pick up permission changes. See the Refresh all menu option in Folders for instructions.
If you apply a new permission to a folder that already has tasks in it, the permissions will not be applied to the tasks until the tasks are recalculated, for example by a change of status. You can force recalculation for all the tasks in a folder and in subfolders. See the Refresh tasks folder-level menu option in Folders for instructions.
You can add permissions for an individual user, but in most cases the best option is to add user group permissions when the folder is created, as that is the easiest to manage.