A problem like this can be a little more involved than you may initially think, due to VB's handling of events. I messed around and came up with this, which I think will do what you want. (You can open a new VB project, slap a listbox with checkbox style on the form, paste in this code, and check it out.)

VB Code:
  1. Option Explicit

  2.  
  3. Dim mblnSuppressClick   As Boolean

  4. Dim mblnAfterItemCheck  As Boolean

  5.  
  6. Private Sub Form_Load()

  7.     

  8.     Dim intX    As Integer

  9.     

  10.     For intX = 1 To 10

  11.         List1.AddItem "Person #" & intX

  12.     Next

  13.  
  14. End Sub

  15.  
  16. Private Sub List1_Click()

  17.     

  18.     If mblnAfterItemCheck = True Then

  19.         mblnAfterItemCheck = False

  20.         Exit Sub

  21.     End If

  22.     

  23.     If mblnSuppressClick = True Then

  24.         mblnSuppressClick = False

  25.         Exit Sub

  26.     End If

  27.  
  28.     mblnSuppressClick = True

  29.  
  30.     If List1.SelCount > 6 Then

  31.         MsgBox "A maximum of 6 people can be selected."

  32.         List1.Selected(List1.ListIndex) = False

  33.     Else

  34.         List1.Selected(List1.ListIndex) = Not List1.Selected(List1.ListIndex)

  35.         If List1.SelCount > 6 Then

  36.             MsgBox "A maximum of 6 people can be selected."

  37.             List1.Selected(List1.ListIndex) = False

  38.         End If

  39.     End If

  40.     

  41.     

  42. End Sub

  43.  
  44. Private Sub List1_ItemCheck(Item As Integer)

  45.     

  46.     If mblnSuppressClick = True Then

  47.         mblnSuppressClick = False

  48.         mblnAfterItemCheck = True

  49.         Exit Sub

  50.     End If

  51.  
  52.     mblnSuppressClick = True

  53.     

  54.     If List1.SelCount > 6 Then

  55.         MsgBox "A maximum of 6 people can be selected."

  56.         List1.Selected(List1.ListIndex) = False

  57.     End If

  58.     

  59. End Sub