Generally, yes. Paratext 8 and 9 use the same project files and are compatible with one another. I used Paratext 9 for some time before upgrading the rest of the team to 9. There were some early versions of PT8 which are less compatible with PT9, so I would make sure that the PT8 users are upgraded to the latest version of 8. You can see the version being used by each member of the project by going to Project Menu> Recent Changes.