»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)
