button visibility in openerp
Depending the logged in user : 
You can use the variable 'uid' but I don't think you can do 'uid.name' or 'uid.groups_id'. So the easier method will be the second.
Depending on the groups
Example : We have some users who are managers and others not, create a group 'Manager' (in a xml file !!!) and add this group to managers. Now change the field in the xml like this :
<field name="name" string="this is the string" groups="my_module.my_reference_to_the_group"/>

The field will only be visible for managers
I am using openerp 5.16 web. 
Is there any way we can hide button depending upon the logged in user. 
or how can i control the group visibility depending upon the user group.