I've received a few emails recently addressed to a relatively large number of people (around 30-40). The email addresses are all in the "to" or "cc" field, so shared with everyone. There's been one from DD's playgroup, which asked us to give us an email address to save on printing costs - I was happy to do that, but didn't realise they were going to share my email with all the other parents. Another was from a music class I'd taken the DCs to in the school holidays; the person running it had my email from when I'd booked and then included me in a mass email about her future classes.
Quite possibly I'm being a bit petty over this and there are other more pressing IT concerns I could worry about. However, I thought it was good practice to BCC people, both for reasons of privacy and to have a tidier email header; this is what I've done in the past when emailing large groups of people.