The Models Menu (available in Developer Mode) tells you which groups are needed for each Document type in Odoo:

The Groups Menu (available in Developer Mode) tells you which groups inherit the permissions of other groups:

So, for access to CRM, users need to be a Member of:

Either:

• Sales / Administrator (read, write, create, delete) 

OR:

• Sales / All Documents (read, write, create)

OR:

• Sales / Own Documents Only (read, write, create)