Personally, I would use bound controls to save the selected values. Data structure looks good. The main form is bound to Assignments and this is essentially backwards. Instead of subform for projects, could have projects as the main form and Assignments would be subform. Review
http://office.microsoft.com/en-us/ac...010098674.aspx
The form could be opened to only allow new project entry or could have search functionality to find existing projects and add employees. Use the intrinsic navigation bar or build custom buttons for navigation.
Also recommend making form smaller, it stretches too wide.