The correct option is B Either we will go to the party together, or I'll meet you at the venue itself.
A compound sentence is made up of independent clauses that can stand on
their own as sentences. Alternative compound sentences express a choice
between two alternate ideas, like in the given sentences.
There is a choice expressed here, "go...together" or "meet at the venue itself".
'Either...or' refers to alternatives; when it is placed in the sentence, the clauses should be parallel in the grammatical structure. (A parallel structure is the repetition of a chosen grammatical form in the sentence, that form being correct in the preferred grammatical sense.)
In B, the clauses are parallel in terms of its structure: subject (we, I) + verb (will go, meet you). Hence option B is correct.
In option A, the error lies in "I'll or meet", this part is grammatically incorrect, hence A is wrong.
Option C is incoherent in meaning; with 'either' being placed before 'together' and 'meet you or at the venue' shows a choice between 'you' and the 'venue', which does not express a complete meaning. Hence option C is incorrect.
Similarly in option D, the sentence isn't complete in meaning, the conjunctions 'either' and 'or' being at the end of each clause. Hence D is also wrong.