»Mitfeiern« means »celebrate with someone else« and it can be used with any grammatical person. If you want to translate it into one english word, you best think of something like *co-celebrate*, in a similar meaning as *co-exist* or *co-work*.

> Ich würde gerne mit euch mitfeiern.  
> I would like to celebrate with you. (plural-you)  
>
> Ich würde gerne mit dir mitfeiern.  
> I would like to celebrate with you. (singular-you)  
>
> Du kannst gerne mit uns mitfeiern.  
> You are welcome to celebrate with us.
>
> Möchte deine Frau auch mit uns mitfeiern?  
> Does your wife also want to celebrate with us?  
>
> Hast du gehört? Hans hätte auch gerne mit Sabine und Ilse mitgefeiert.  
> Did you hear? Hans also would have liked to celebrate with Sabine ans Ilse.  


btw: In your example you did not translate the sentence correctly. This is a translation, that is closer to the original meaning:

> Schade, dass wir nicht mitfeiern können.  
> Shame, that we can't celebrate with whoever is celebrating.  

(The context of the german sentence doesn't tell you who is celebrating. It could be *you*, or it could also be *them*)

> Shame we can’t be there to celebrate with you.  
> Schade, dass wir nicht dort sein können um mit dir/euch zu feiern.  

(The english context doesn't make clear, if you is singular or plural)