How to allow an user of a group to modify specific parts of a form in Odoo?
You cannot make few or some of the fields as 'readonly' in odoo based on the groups. If you need to do it, you can use the custom module 'smile_model_access_extension'. 
For loading appropriate view on menu click you can create record of 'ir.actions.act_window' (view_ids) field of 'ir.action', where you can specify the sequence and type of view to be loaded when the menu action is performed. In your case you can specify the specific 'form view' for your action.
I've created a new group named accountant. If an user of this group opens the res.partner form for example, he must be able to read all, but only modify some specific fields (the ones inside the tab Accountancy, for example).
So I set the permissions create, write, unlink, read to 0, 1, 0, 1 in the res.partner model for the accountant group.
The problem: if I'm an user of the accountant group and I go to the form of res.partner, I will see the Edit button, if I click on it, I will be able to modify any field I want (and I should not, only the ones inside the tab).
So I thought to duplicate the menuitem (put the attribute groups="accountant" to the copy) and the form (put all fields readonly except for the content of the tab).
The problem: if I'm an user of a group over accountant group (with accountant in its implied_ids list), I will see both menuitems (the one which takes to the normal form and the one which takes to the duplicated form with the readonly fields).
Is it possible to create a menuitem which opens a specific set of views depending on the group of the user who is clicking on the mentioned menuitem? Any ideas of how can I succesfully implement this?