[hokoto]

I think the party host who is sending his invitations without using bcc is to blame here.

I use gmail mainly for business related mails and the default reply-all makes a lot sense - almost all the time other recipients ( managers, clients etc) are there to be kept in loop. If you forgot and just reply to it, then you have send the same mail again - or worse you never notice it.

[sssparkkk]

The party host is not using bcc because he/she wants everyone to be aware of who has been invited to the party.

I guess it boils down to your default usage of email; I've never had an email client before Inbox that does 'reply all' by default.