If you're developing a module, you can put a file named ir.model.access.csv inside your security folder (include it in the data section of your __openerp__.py) with the following layout to create ACL's

(In this working example, all users from four groups can read from that object, but only manager can read, write, create, unlink. More explanation on this subject here: https://www.odoo.com/forum/Help-1/question/What-are-these-in-irmodelaccesscsv--12430)

id,name,model_id:id,group_id:id,perm_read,perm_write,perm_create,perm_unlink
access_generic_request_user,generic_request.user,model_generic_request,group_processos_user,1,0,0,0
access_generic_request_logistics,generic_request.logistics,model_generic_request,group_processos_logistics,1,0,0,0
access_generic_request_hr,generic_request.hr,model_generic_request,group_processos_hr,1,0,0,0
access_generic_request_manager,generic_request.manager,model_generic_request,group_processos_manager,1,1,1,1

If not, you can create these rules accessing the Settings »» Technical »» Security »» Access Control List option in OpenERP.