Requestors end users will not acces a site directly and add list items

Hi

i created a  site for IT services  and this site includes some lists for various services

http://Intranet.com/Units/IT services 

main site is http://Intranet.com

and also workflow are configured  for these lists for create new item

and only 3 type of users will work on these lists

1)Requestors (Create,View rights)

2)Contributers (Contribute permissions)

3)Approvers

Scenario:

-- Requestors    will create list items and approvers will Open this list item and  edit list item like

 approve and add a Contribute userid in AssignedTO field

---Contribute user also will recieve email and he also Open item and edit ans save this list

so basically its backend  , data inserted by requestors will  save in lists

so here i need all  Requestors  end users will not acces IT services site and add list item directly in list.

is any other option availabe that i will provide a  diffrent front end page for Requestors at root level  site (which is Intranet.com )

and they will add and view only their data, and  Contributers  also  view and edit all users data.

adil