Grouping option

Shruthi96  can you try this

let
    Source = Csv.Document(File.Contents("C:\Users\user\Desktop\source\Book3.csv"),[Delimiter=",", Columns=4, Encoding=1252, QuoteStyle=QuoteStyle.None]),
    #"Promoted Headers" = Table.PromoteHeaders(Source, [PromoteAllScalars=true]),
    CT = Table.TransformColumnTypes(#"Promoted Headers",{{"Task", type text}, {"Assignment group", type text}, {"Start", type datetime}, {"End", type datetime}}),
    src=CT[Assignment group],
    L2 = List.Generate(
                           ()=>[i=0, j=src{i}, k=try src{i-1} otherwise j, l=1],
                               each [i]<List.Count(src),
                               each [i=[i]+1,j=src{i}, k=try src{i-1} otherwise j, l=if k<>j then [l]+1 else [l]],
                               each [l]
    ),
    ColNames1 = Table.ColumnNames(CT),
    L1 = Table.ToColumns(CT),
    ColNames2 = List.Combine({ColNames1,{"Grouping"}}),
    Custom1 = Table.FromColumns(L1&{L2},ColNames2),
    #"Grouped Rows" = Table.Group(Custom1, {"Task", "Assignment group", "Grouping"}, {{"ad", each _, type table [Task=nullable text, Assignment group=nullable text, Start=nullable datetime, End=nullable datetime, Grouping=number]}}),
    #"Added Custom" = Table.AddColumn(#"Grouped Rows", "Start", each let x =[ad] 
    in List.Min(x[Start])),
    #"Added Custom1" = Table.AddColumn(#"Added Custom", "End", each let x =[ad] 
    in List.Max(x[End])),
    #"Removed Columns" = Table.RemoveColumns(#"Added Custom1",{"ad"})
in
    #"Removed Columns"