You'll need a new table with; UserID, a date/time field and an In/Out field. Make the date field's default value =Now(). The In/Out field could be a 3 digit text field.
strSQL = "INSERT INTO tblUserLog ( UserID, InOut )"
strSQL = strSQL & " VALUES ( " & [TempVars]![CurrentUserID] & ", 'In' ); "...