are you down for..

"Are you down for Friday's event?" would usually mean "Have you been selected to participate in Friday's event?"

"Are you up for Friday's event?" would usually mean "Are you willing to attend / enthusiastic about attending Friday's event?"

Huzaifa AsifAre you down for friday's event?
Is the sentence above same as saying
Are you up for friday's event?

For some speakers, yes, in the sense “Do you want to attend Friday’s event?” It’s slang, though, while up for is just informal.

Aspara GusFor some speakers, yes, in the sense “Do you want to attend Friday’s event?”

So you're saying that "Are you down for Friday's event?" can mean "Do you want to attend Friday's event?"? I don't know that meaning.

GPYI don't know that meaning.

I don’t know how common it is in BrE. But I do know it’s used more by younger than older generation speakers, and comes from African American English.
The for phrase is often omitted: I’m going out. Are you down?

Aspara GusThe for phrase is often omitted: I’m going out. Are you down?

Oh, OK, thanks, I've never heard of it.

Yes but is the phrase 'are you down for..' recognized by many or I'm afraid many of them still consider it to be wrong in terms of grammar?

I, a speaker of BrE, understand it only in the sense that GPY noted.

Huzaifa AsifYes but is the phrase 'are you down for..' recognized by many or I'm afraid many of them still consider it to be wrong in terms of grammar?

The meaning that I explained is generally accepted and understood, as far as I know. The meaning that Aspara Gus explained is slang that seems to be restricted to certain speakers, possibly only in the US. The