The word "just" can have two meanings:
1) A few moments ago
It depends whether the meaning of the sentence is "I saw only two people at the party. Those two people were Meg and Brian" or if it is "A few moments ago, I saw Meg and Brian at the party".
It's a poor quiz/test question anyway because in most variants of English, we would put "just" after "I" (or after "I have") if we meant "a few moments ago".
I just saw Meg and Brian at the party.
I have just seen Meg and Brian at the party.
If we want it to mean "I saw only two people", then it would be":
I saw just Meg and Brian at the party.
I would find that version very unnatural with "have seen".
Note my amendments to your post in red. Remember to use just one full stop at the end of a sentence. There is no need to capitalise the names of the tenses.
Student or Learner