My guess would be that your Trello was created with an Atlassian account that was tied to an organisations email, and the organisation has claimed the domain, and enabled access to the rest of the organisation.
If for example, I had signed up for a Trello using my Adaptavist email address, then when Adaptavist claimed the adaptavist domain, my trello would then have all my colleagues in it, because it belongs to the adaptavist domain. (Doesn't necessarily mean they'd have any access to my trello, but they would be valid users)
It's either that or one of the other people you'd shared it with have invited lots of people in.
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