The english verb to meet is translated into a reflexive verb in German, if it is not used with another object: sich treffen. (Whereas treffen, the non reflexive verb, means to hit, to strike.)
So, for example:
I meet you → Ich treffe dich (or Ich treffe mich mit dir.)
We meet → Wir treffen uns.
You could understand (and remember) this easily, if you take it as if you would say We meet each other instead of We meet in English. For the German verb treffen (in the sense of to meet) the object is mandatory, and the reflexive pronoun takes the role of the object.
In your sentence, uns is the reflexive pronoun in first person plural, and since there is no other object, the reflexive pronoun must not be omitted.
mal (its literal translation would be once) is a filling phrase adding some vagueness about the time. Its translation is discussed at Looking for extensive discussion of German modal particles. Anyway, it could be omitted from a grammatical point of view.
The preposition and article in zu einem Essen would be shortened to zum Essen. The difference is somewhat similar to the difference between Maybe we can meet for a dinner. or Maybe we can meet for dinner. in English.
So possible translations are:
Vielleicht können wir uns zum Essen treffen.
Vielleicht können wir uns mal zum Essen treffen.