Engagement Party Invitations

Hmmm I think in this case you don’t have to

Couples are a social unit and should be invited as such.

I agree with both comments above - if the "plus one" is a significant other of the person in the wedding party, then definitely invite them, since couples are a social unit and both should be invited. If the wedding party member is single, I don't think you necessarily need to give a plus one. If space and budget allow you to give a plus one to the single wedding party members, it would be a nice thing to do, but if it's not in your budget, I think it's OK to not give them a plus one to the engagement party.

I think the spouse/partner should be included.

If someone in the bridal party is in a relationship or married, yes, the significant other should be invited. Significant others aren’t plus ones. Plus ones are dates/friends of single guests, and while the single members of the bridal party should get one for the wedding (even if other guests do not), they don’t need one for the engagement party.