Laura, this sounds like a maintenance nightmare!

You need Access to calculate this for you at runtime, and you could achieve
that if you had these 3 tables:

Company table:
   CompanyID    primary key

Employee table:
   EmployeeID    primary key

CompanyEmployee table:
   CompanyID    foreign key to Company.CompanyID
   EmployeeID    foreign key to Employee.EmployeeID

Now create two queries:

1. Create a query into the CompanyEmployee table.
Drag a 2nd copy of the CompanyEmployee table into the grid.
Access will alias it as CompanyEmployee_1.

2. In the upper pane, create a join between CompanyEmployee.EmployeeID and
CompanyEmployee_1.EmployeeID. If you see a join between
CompanyEmployee.CompanyID and CompanyEmployee_1.CompanyID, delete that line.

3. Drag CompanyEmployee.CompanyID into the query grid.
Type this into the Field row:
   CommonCompanyID:[CompanyEmployee_1].[CompanyID]
In the Criteria row under this field, enter:
   <> CompanyEmployee.CompanyID.

4. Save the query with the name qryEmployeeBoth. Close.
This query shows each matching company that has shared employees, and lists
who they are.

1. Create a new query into the Company table and also the query you just
saved. If you don't see a line joining Company.CompanyID to qryEmployeeID,
then drag the one field onto the other. Double-click the join line. Access
pops up a dialog offereing 3 choices. Choose the one that says:
   All records from Company, and any matches from ...

2. Depress the Totals button on the toobar.
Access adds a Total row to the grid.

3. Drag Company.CompanyID and qryEmployeeBoth.CommonCompanyID into the grid.
Accept Group By under these fields.

4. Drag qryEmployeeBoth.EmployeeID into the grid. In the Total row, choose
Count.

This query lists every company matched against any other companies that have
shared employees, and the number of shared employees.

This willbe the SQL for the 2 queries:
SELECT CompanyEmployee.CompanyID, CompanyEmployee.EmployeeID,
CompanyEmployee_1.CompanyID AS CommonCompanyID
FROM CompanyEmployee INNER JOIN CompanyEmployee AS
CompanyEmployee_1 ON CompanyEmployee.EmployeeID =
CompanyEmployee_1.EmployeeID
WHERE CompanyEmployee_1.CompanyID <> [CompanyEmployee].[CompanyID];

SELECT Company.CompanyID, qryEmployeeBoth.CommonCompanyID,
Count(qryEmployeeBoth.EmployeeID) AS CountOfEmployeeID
FROM Company LEFT JOIN qryEmployeeBoth
ON Company.CompanyID = qryEmployeeBoth.CompanyID
GROUP BY Company.CompanyID, qryEmployeeBoth.CommonCompanyID;