In an earlier lesson we've already learned that in the plain form, we can drop です after い-adjectives, and that we can change です into だ after nouns and な-adjectives.

In casual conversation with people we know well, we can even drop the だ after nouns and な-adjectives.

In the last sentence above, it would be grammatically wrong to use to use だ in front of the question particle か. Though です is frequently used in front of the question particle, it is mandatory to drop だ when using the question particle.

Be careful with using か without using the copula です, because in the wrong situation, it can easily sound excessively harsh or rude.

In casual speech with close friends or people lower in status, it's possible to use the plain form in invitations.

As you can see in the last sentence, invitations can be made even more informal by dropping the か. And while we're at it, why not drop the object particle after ビール as well?

In casual speech in general, we can drop the particle か in question sentences, and instead use a rising intonation at the end of the sentence.

Breakdown of daijoubu.