You can add a custom field to project.project.

'customer_team_ids': fields.many2many('res.partner', 'project_partner_rel', 'project_id', 'partner_id', 'Customer Team')

<field name="customer_team_ids"/>

Either using the webclient or a custom python module.

EDIT:

If you want to avoid programming, you can use the webclient to add the field.

Log in as admin with Technical Settings access. Go to Settings -> Technical -> Database Structure -> Models. Select the project.project and add a new many2many field with Object Relation 'res.partner'.

Next go to Settings -> Technical -> Database Structure -> User Interface. Select the project form view (I think it is 'project.edit_project') and duplicate it. Rename your duplicate, set the Inherited View to the view you've just duplicated and add the field where you want it.

Hope that helps.

Regards.