"My dog is playing with you all."
Translation:Mein Hund spielt mit euch.
ihr is "you (plural)" in the nominative case -- but here, the preposition mit requires the dative case.
Saying mit ihr would be grammatically as wrong as "with he" or "with I", which would have to be "with him" and "with me" in English.
It is unfortunate for learners of German that "you" in English not only doesn't distinguish between singular and plural, but not between subject and object form, either.
But Hund is masculine, right?
So shouldn't it be "Meiner Hund" instead of Mein Hund which is nominative for a neuter noun?
Possessive determiners such as mein and euer inflect like the indefinite articles ein and kein — which means that they have no ending in masculine and neuter nominative.
We say ein Hund, mein Hund and ein Pferd, mein Pferd rather than einer Hund, meiner Hund or eines Pferd, meines Pferd.
(On the other hand, we do use an ending there when the words are used as pronouns rather than determiners: Dies ist ein Hund und das ist auch einer; dein Hund ist groß aber meiner ist klein. Compare the English where such words as “mine” and “hers” are also longer than “my” and “her”.