Hi Daniel,

what is overriding "onchange_project(project_id)" method and adding: domain for user_id there? you have the project_id so you can load the members and can add the list in return

As i read https://doc.openerp.com/6.0/developer/2_6_views_events/events/events/ correctly onchange methods can return not only values and warning but also "domain"

EDIT i just test my idea with a short modul for OpenERP 7 and it works, look at the code:

from openerp.osv import fields, osv
from openerp.addons.base_status.base_stage import base_stage

class task(base_stage, osv.osv):
    _name = "project.task"
    _inherit = "project.task"

    def onchange_project(self, cr, uid, id, project_id, context=None):
        if project_id:
            project = self.pool.get('project.project').browse(cr, uid, project_id, context=context)
            members = []            
            for m in project.members:
                members.append(m.id)
            if project and project.partner_id:
                return {'value': {'partner_id': project.partner_id.id},
                        'domain': {'user_id':[('id','in',members)]}}
            return {'domain': {'user_id':[('id','in',members)]}}
        return {}

task()

Greetings Markus