Translation:I will have a party tomorrow with my friends.
I've been getting this same exact sentence, and the one time it marks "going" as wrong and says "have" is right, and the other does it the other-way around - so there's no way I can be sure to get it right. I double-checked and copied the characters: the Japanese is identical, then copied the answer word-for-word. This is, to put it mildly, frustrating.
Hoovering over it suggests: "with", "and" (as we learned earlier), "to"
Edit: Corrected my previous "answer" after confirming with later phrases.
と is indeed being used to say "with" and is basically referencing a omitted 私 as is so natural and common in the language.
If not for the と the meaning would be that the friends were going/having the party themselves with poor私 not included.
を comes after "party" marking it as the direct object that will "suffer" the action of the verb "having/going the party" done by 私と友だち
(私と友だち is just for illustration purposes, google translates it as expected though as a learner I still have no confidence to tell you this won't be wrong and/or unnatural sounding but it's SOUND by logic xD)
You probably already figured it out but might as well put it here for posterity :P
I keep getting this answer wrong. I suspect that it has something to do with how I type out パーテイー, since my イ isn't diminutive (I think the word is digraph?) like Duo's answer is: パーティー. When I copy and paste the two into Google Translate, it shows me that the Romaji I'm writing is pāteī vs. Duo's pātī. However, I wasn't aware of tī being a digraph. How do I type out this in katakana on the keyboard-- how do I make the イ small?