Learn English – “we met at a party” vs. “we didn’t meet at a party”

Your second sentence is great. I thought that if he did that, he would be late to the party.

"Last night I thought that if he did that, he would be late to the party."

Adding in a time frame can help you determine the tense.

This expresses the idea that "you", sometime in the past, had thought about the consequences of the other persons actions, and concluded he would be late to the party.

Some explanations in this answer are going to be unorthodox, but all of them are in good faith. They are there to help you shatter your current understanding that gets in your way. (Also, keep in mind that I'm not trying to be technically precise here, but I'm more on the practical side of it.)

Let's lay the groundwork.

First, will is not the future tense. Your example sentence, Some of you will have met me before, is a good example showing why we should break away from the concept of three time zones (past, present, and future) and move toward the two-tense system (past and non-past), and why we should treat will as a modal verb, like all its friends (can, may, should, and so on). If you don't think of "future" every time you see will, it'll be easier for you to understand your example.

Second, your usage directly reflects your thoughts, your views, not situations. A typical grammar book will explain to you, we use this or that tense for a) ..., b) ..., c) ..., and by doing so, it gives the reader (a learner) the impression that in the case of a) or b) or c), we have to use that tense. Not so! This is a typical problem. The book is technically not wrong, it's true that in the case of a) or b) or c), we can use that tense. We (or most of us, the learners, anyway) arrive at the "have to" conclusion by ourselves. Most of us understand grammar backward: in this type of event, we have to use this tense, aspect, etc. Or if we find this tense, aspect, etc., in a sentence, the event must be of this type. How can we fix this? One possible way is to think of these grammar topics as "features" of a language, and they're used to give you, the listener or the reader, some clues, what and how the speaker thinks of the event. It's not necessarily identical to the actual event, but that's how the speaker thinks of it.

Third, let's consider will a little bit more. Let's consider it as a modal verb, not the "future" tense. The modal verb will can be used (according to Geoffrey N. Leech) to convey prediction (or predictability), willingness, insistence, and intention. Here are some prototypical examples:

By now they will be eating dinner. (prediction)
Will you please open the door for me? (willingness)
I will go to the dance! You can't stop me! (insistence)
I'll write tomorrow. (intention)

I think we're ready now.
Let's get back to your sentence: Some of you will have met me before.

You wonder why it wasn't Some of you (have) met me before. This suggests that you understand the simple past and the present perfect well. Indeed, both of them are grammatical, though Some of you have met me before is a better choice because it's more logical to think that the speaker is recalling indefinite experiences, hence the experiential perfect. Let's stick with Some of you have met me before because it's closer to the original, Some of you will have met me before.

If we consider will as a modal verb with four main uses (prediction, willingness, insistence, and intention), it's clear that in your example sentence, the only sensible use of this will is prediction. Paraphrasing the sentence in contrast with may have been should make this point clear:

Some of you may have met me before. (= It's possible that some of you have met me before.)
Some of you will have met me before. (= It's predictable that some of you have met me before.)

(Note that these paraphrases are only meant as understanding aids, not formal interpretations.)

I think you should be able to answer the remaining question now. Is it possible to use Some of you will have met me by tomorrow? In case you're not sure, it's Yes! :-)