You need the following privileges to generate models from already existing objects:
You need the Read access privilege for all groups containing objects that you want to select for model generation.
For all groups with models that you want to select for model generation, you will need the Read access privilege.
For all groups in which you want to save a generated model, you need the Write access privilege.
During creation of object occurrences, your access privileges for the source objects will not be checked. Occurrences will also be created from object definitions saved in a group for which you do not possess the Read access privilege. These are shaded in gray.