Everyone is suddenly very interested in WebMeeting.
You can block port 5001, that's fine. You'll need to allow communication to webmeeting.3cx.net on port 443. The whole WebMeeting platform is located there, not on your PBX. This is why a communication channel between the two is required in order for the platform to know how to setup your meeting.
If you won't tell webmeeting.3cx.net who is invited to your meeting, how does it know who is invited and what his name is in order to show it in the participant pane;?
Another example is, if you don't tell webmeeting.3cx.net what your meeting title is, how can it set a title for your meeting?
There's load of examples i can make, but i hope the above give you the point on why the 5001 to webmeeting configuration is required.
Now in regards to the issue you're facing with the inviting of participants, having port 5001 open to webmeeting.3cx.net is a requirement and it wont work without because of the following:
1) You connect you a WebMeeting room. This means you are connected to webmeeting.3cx.net.
2) You invite someone via the in-built invite function inside the meeting room.
3) webmeeting.3cx.net tells your PBX via it's MyPhone port that it needs to send an e-mail invite to X user.
4) If the port is closed, the mail send fails. If the port is open, the mail send succeeds.
WebMeeting doesn't send any e-mails directly to any user. It instructs your PBX to send an e-mail. WebMeeting will not be responsible for sending your own e-mails, it makes more sense that you send them. This gives you more control over them.
I hope this answers your question.