You can add a line in the ir.model.access.csv of the concerned module like that :

id  name    model_id:id group_id:id perm_read   perm_write  perm_create perm_unlink

access_hr_th_group_thw_emp  hr.th.employee_id       hr.model_hr_th  group_thw_emp   1   0   0   0

I'm not sure about my line cause i don't know your groups and module, but you can inspire yourself with any ir.model.access.csv on the base code.

EDIT :

Here some links about ir_model_access :

https://accounts.openerp.com/forum/Help-1/question/12430

In this link you can read :

For :

id  name    model_id/id group_id/id perm_read   perm_write  
perm_create perm_unlink
model_bpl_worker_manager    bpl_worker  model_bpl_worker    
group_checkroll_manager 1   1   1   1

• id = unique identify for the permission (Example: MY_MODULE_res_partner_manager)
• name = unique name for the permission (Example: res_partner manager)
• model_id/id = the model unique name of the class you want apply permission on (Example model_res_partner)
• group_id/id = group permission apply on (yopu can define it in xml group file or call an existing group with syntax module.group_id)
• perm_read,perm_write,perm_create,perm_unlink = the 4 values for the relative permission to read, write,create,unlink record on defined class. 1 is True (you can do this action) and 0 is Faslse (you can't)

A simple way to understand this file is to read an existing csv in some OpenERP base module like sale, account, product, etc...

Some useful link: http://www.zbeanztech.com/blog/security-openerp

I can give you others links likes :

http://forum.openerp.com/forum/topic16596.html

http://stackoverflow.com/questions/15966676/openerp-7-access-rights-for-user-roles