In my experience, yes!

But the problem is not so much them adding without asking, it's the fact that the SaaS companies are making random assumptions regarding who owns the email account, or who owns the domain name of those accounts, and not letting users know that before accepting. Apparently only gmail/hotmail/yahoo are safe, unfortunately.

A few years ago I tried to sign up to LinkedIn only to find out I already have an account. But it was not my account - it was someone else's, who has a similar name (and apparently thought my email address was his? I don't know...). I could reset the password and log in to his account. I was a bit scared when I contacted LinkedIn support, because I was worried they would accuse me of hacking. Eventually they disconnected my email from his account.

Now, I know that LinkedIn isn't a SaaS platform, but you would think that such a big company wouldn't make such rookie mistakes. Even if they launched without email confirmation, and added it later, there should be some mechanism to "fix" the old accounts.