This code will remove all the 'joined the meeting' paragraphs.
Code:
Sub RemoveJoin()
Dim oRng As Range, iType As Integer
Set oRng = ActiveDocument.Range
With oRng.Find
.ClearFormatting
.Text = "joined the meeting^p"
While .Execute
oRng.Paragraphs(1).Range.Delete
Wend
End With
End Sub
You could shorten the search string it to remove 'joined' to get rid of both in one pass but you might lose some desired paragraphs in friendly fire. You could also repeat the full code to get the 'left the meeting' paragraphs.