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I was able to re-create the same problem on our client installation and on the demo site.
Below are the step-by-step instruction how that happened.

What seems to be a crucial difference in my several tests (and possible cause of that problem) is the groups relationships: when the relationship parent-child of the groups was assigned AFTER there were members assigned to those groups, that causes incorrect number of participant added to the event. Complicated? Let me explain step by step, based on what I did on the demo installation.

1. Create new groups "Test Group for bug CRM-4270", "Child 1 of Test Group for bug CRM-4270" and "Child 2 of Child 1". DO NOT assign parent information at this moment yet - this is a crucial difference between this test that results in incorrect number of event participant and the test when everything worked OK - more notes on that below.
2. Add to the group "Test Group for bug CRM-4270" 4 random contacts, that are not members of any other group.
3. Create 2 contacts:
- "Contact1 ForBugTest" with membership in all 3 groups (Test Group for bug CRM-4270, Child 1 of Test Group for bug CRM-4270,  Child 2 of Child 1)
- "Contact2 ForBug" with membership in 2 groups (Test Group for bug CRM-4270, Child 2 of Child 1).
4. Modify groups relationships, added "Test Group for bug CRM-4270" as a parent of "Child 1 of Test Group for bug CRM-4270" and "Child 1 of Test Group for bug CRM-4270" as a parent of "Child 2 of Child 1".
5. Go back to "Test Group for bug CRM-4270", view members.
6. Select records -> All 6 records.
7. Select action "Add Contact to Event", GO.
8. Add contact to MyMeeting event that is empty (have no participants).
9. The result: 9 participants added.
As you can see above, there are only 6 participants in that group BUT one contact is a members in 2 groups and one contact in 3 groups.
When I check MyMeeting participants, I can see that the contact that was a member of 3 groups (Contact1 ForBugTest) was added 3 times and "Contact2 ForBug" was added twice.

When I repeated this test, but in the step 1 created group relationships right away (assigned parents right away, before adding members to the groups), the were no errors in adding participants to the event (as many as the group had members, that many participants were added to the event).

My client installation has the same situation. Groups were created and members were added and only after that the relationships between groups were created - and they have exactly the same problem with adding all members of the group as event participants.

At the moment the solution for us is to create new groups, assign proper relationships and only after that add group members - that solves the problem, when we want to assign all group members to the event.

I hope that will help narrow down the issue and find a solution.