It's a bug. Users with permissions to read Tasks should be shown too. You can correct it by editing line 417 of file 'application/helpers/permissions.php', from:
if ($proj_perm && can_manage_type($object_manager,$proj_perm, ACCESS_LEVEL_WRITE)){
to:
if ($proj_perm && can_manage_type($object_manager,$proj_perm, ACCESS_LEVEL_READ)){
It will be fixed for next version.
Thanks.