Problem Setting Up Relationships

I have a table which contains a list of names and the trades that each one is qualified for and a second table for a project which requires a number of different trades to fill specific roles. How do I set up a proper relationship between the two tables.

Tbl_employee.
empl_ID| Empl_Name | Discipline
001 | Mathew | Construction Manager
002 | Mark | Safety officer
003 | Luke | Construction Manager
004 | John | Architect

Tbl_Proj.
Project_ID
Project_name
Project_Architect
Project Manager
Health & Safety

Each employee could have more than one role on each project, one role on each project, or no roles on a project.

I have a form that shows a complete record for each project and permits me to select who fills what roles from a combo box for each role.

Using the relationship editor I try to create relationships between the following:

Discipline - Project Manager
Discipline - Project_Architect
Discipline - Health & Safety

I then set up a SELECT in the rowsource of each combo box on the form WHERE the Discipline is whichever is required by the combobox

The raltionship editor allows the creation of a relationship between Discipline and the (multiple) various roles on the project, but if I try to 'Enforce Referential Integrity' It refuses to work.

Do I need 'Referential Integrity' and if so How do I make it work?

Thx,
Kev.