Etiquette used to state that you weren't to invite anyone to the engagement party whom you weren't inviting to the wedding, case closed. But now more couples live and/or host their nuptials far from their families and friends, and the formality of engagement parties is evolving, so expectations have changed. Engagement parties can now often include people who aren't invited to the wedding. If your friends want to plan an informal party at a neighborhood bar and email the invites a few weeks before, it's totally fine to include people you aren't sure will end up making the wedding guest list (coworkers, newer friends). And if your parents' good friends want to host a cocktail party at their home in your honor, let your parents invite mutual friends and business associates you might not have room for at your wedding.
However, if you two or your parents are hosting, the old rule sticks: When the wedding hosts send the engagement party invitation, it's considered part of the official wedding parties and guests assume they're invited to the wedding too. To avoid a sticky situation later, start working on your wedding guest list now. Then trim the engagement party list down to your wedding party, immediate family and closest friends.