MIN & MAX date in group where group repeats

Need a group identifier. Using the given sample: the first 3 records are group 1, next 2 are group 2, last 2 are group 3. Use VBA behind Excel to populate a column. The group ID changes when the Contract value changes.

Sub SetGID()
Dim c As Integer, r As Integer, x As Integer, i As Integer
c = 1
r = 2
i = 1
x = Cells(r, 4)
Do While Not IsEmpty(Cells(r, 2))
    Cells(r, 1) = i
    r = r + 1
    If Cells(r, 4) <> x Then
        i = i + 1
        x = Cells(r, 4)
    End If
Loop
End Sub

Or import the data as is to Access and run similar procedure to update a field in table. This would involve opening a recordset and looping its records.

Then aggregate query summarizes on the GroupID.

SELECT          GroupID, EmpNo, Contract, 
                MIN(WorkDate) AS StartDate, 
                MAX(WorkDate) AS EndDate
FROM            RD1
GROUP BY        GroupID, EmpNo, Contract  

One point I missed from my question, which is fairly important, is that when an EmpNo starts they will have sequential dates up to when they finish with no missing dates. With this in mind you can join the table to itself to return the previous and next day figures.

RD1

Query8

The first query will have two joins. The first joins the table with itself so 14-Mar-21 is connected to 15-Mar-21 and so on. The second joins the table with itself again so that 14-Mar-21 is connected to 13-Mar-21, etc.
With these joins you can check if the current Contract value is different from the previous or next days value - therefore identify the start and end of each block.
(I could probably tidy up the WHERE clause a bit)

SELECT  RD1.WorkDate,
        RD1.EmpNo,
        RD1.Contract,
        IIF((RD1.EmpNo = RD2.EmpNo OR ISNULL(RD2.EmpNo)) AND 
            (RD1.Contract <> RD2.Contract OR ISNULL(RD2.Contract)),'Start') AS StartID,
        IIF((RD1.EmpNo = RD3.EmpNo OR ISNULL(RD3.EmpNo)) AND
                 (RD1.Contract <> RD3.Contract OR ISNULL(RD3.Contract)),"End") AS EndID
FROM    (RD1 LEFT JOIN RD1 RD2 ON RD1.EmpNo = RD2.EmpNo AND
                                  RD1.WorkDate = RD2.WorkDate+1)
             LEFT JOIN RD1 RD3 ON RD1.EmpNo = RD3.EmpNo AND
                                  RD1.WorkDate = RD3.WorkDate-1                           
WHERE   NOT ISNULL(IIF((RD1.EmpNo = RD2.EmpNo OR ISNULL(RD2.EmpNo)) AND 
                       (RD1.Contract <> RD2.Contract OR ISNULL(RD2.Contract)),'Start',
                       IIF((RD1.EmpNo = RD3.EmpNo OR ISNULL(RD3.EmpNo)) AND
                           (RD1.Contract <> RD3.Contract OR ISNULL(RD3.Contract)),"End")))  

Query9
The next query joins each start date to each end date providing the start date is less than or equal to the end date.

SELECT  T1.EmpNo, 
        T1.Contract, 
        T1.Start, 
        T2.End
FROM    (
        SELECT  EmpNo, Contract, WorkDate AS Start 
        FROM    Query8 
        WHERE   NOT ISNULL(StartID)
        )  AS T1 LEFT JOIN 
        (
        SELECT  EmpNo, Contract, WorkDate AS [End] 
        FROM    Query8 
        WHERE   NOT ISNULL(EndID))  AS T2 
        ON (T1.Start<=T2.End) AND (T1.EmpNo = T2.EmpNo)  

Query10
The final query groups the data by EmpNo, Contract and Start and takes the minimum end date for each group.

SELECT      EmpNo, 
            Contract, 
            Start, 
            Min(End) AS MinOfEnd
FROM        Query9
GROUP BY    EmpNo, Contract, Start
ORDER BY    EmpNo, Start