It is only critical to let the other members of the meeting know that you will be late as soon as you realize that you will be. And you must come to this realization (and let the other attendees know) before the meeting starts, through whatever channel will get to them the fastest. Ideally, you’d let them know about the delay before they must break away from whatever they are doing before the meeting.
In addition to being on time, you must also be present.
Being present means that you are composed, prepared, and focused on the subject matter. It can take a few minutes to “get present” — prepare the agenda, research the topic and the attendees, etc.
Therefore, I recommend that you plan to arrive at an outside meeting fifteen minutes before it is to begin. For a meeting in your office, wrap up your current project five minutes before the scheduled meeting time.
To make this easy, I recommend scheduling 25 and 50-minute meetings only (Google Calendar even has an automated setting for this). This buffer will give you five minutes each half-hour and ten minutes each hour to gather yourself.
When in a meeting, some people have the tendency to make the mistake of constantly checking their messages. They cannot get away from being “on,” if even for a second.
This habit is disrespectful, but it also defeats the purpose of the meeting, which is a collaboration with the attendees present. It sends a message that the meeting’s content is relatively unimportant. Furthermore, it also breeds a bad habit for the entire company—one that will be hard, if not impossible, to break down the line.