If you ask me to get together on a one to one basis, then decide to invite other people, I think you should really tell me. Because that's not the deal I signed up to. I didn't say yes to a group outing or a party. I said I'd catch up with you, singular.
I may not be up for a larger event. Not everyone is a more the merrier kind of person. I'd like to be able to factor that into my decision.
Even adding one other person change the dynamic. Even if they are the nicest person in the whole world. Because there are things I won't talk about with someone I don't know.
ETA - removed a repeated sentence.
ITA. I am a very social person, but I want to know the nature of an event. Is it small, intimate, which is only a few people, or is it a large group, including some people I don't know well. I think it's only polite to tell people stuff like this. Several years ago my then-DH and I were invited to a birthday party by a couple we knew from church; it was their son's 4th birthday. We went, only to discover that we were the only
guests who were non-family. That might not seem odd, except that we knew none of them, and the hosts invited us to "off set" the toxicity (her words) of the family.