Hello Eve,
Sorry for not replying you promptly. I further tested it on my side and found that if we share the live link to other people, they can join in the live but can only view it, they will not appear in participant list, here's what they will see when in live
event:

If we invite them when setting up the live event, they can join the live and will appear in participant list, they can also add other participants:


If that match your current situation, I’m afraid it might be a normal behavior and can’t modified. I’m sorry for that may not the answer you wanted and am totally understand the inconvenience especially when need to manually add 100+ attendees only to let
them show up in list. I’d suggest you submit a feedback at
Teams UserVoice to let our related team know your requirements. We sincerely appreciate your time and understanding. Besides, you may
create a team group in Teams for these attendees, let them join your team, then you can schedule meeting directly in this group, all group members joining the meeting will show up in participant list.
Best Regards,
Anna