I predict a problem with your code (I might be wrong).

The boolean should be Static...

Code:
Private Sub Chat_Change()
    Static TextChange as Boolean
    If TextChange = True Then
        Chat.Text = Chat.Text & vbNewLine
        'Auto Scroll
        Chat.SelStart = Len(Chat.Text)
        TextChange = False
    End If
End Sub
Or Global...

Code:
Option Explicit
Private TextChange as Boolean
Private Sub Chat_Change()
    If TextChange = True Then
        Chat.Text = Chat.Text & vbNewLine
        'Auto Scroll
        Chat.SelStart = Len(Chat.Text)
        TextChange = False
    End If
End Sub
ExcalibursZone is also right. However, if you take the line
Code:
Chat.Text = Chat.Text & vbNewLine
out, you will have to add a line break before adding the next person's chat text.