Hi Julien,
Before I answer your questions, can you please provide more information about what kind of license and permissions do your external project members have. Also what role do they have in that project. Are they resources on tasks or only followers? This way I will be able to focus on what to suggest.
See more information here:
http://usermanual.clarizen.com/#1-4-1-about-user-types/roles
For further clarification, we suggest you register for one of our daily Q&A webinars (see the link-> http://www.clarizen.com/live-demo-europe.html ).These are great resources for free consulting with our Customer Success Managers and visual answers to your questions. One of our experts will be able to discuss your requirements and accordingly suggest best practice advise in detail in a much faster manner.
Thanks!
Diana