كل من المشارك والمتحدث في مقر المؤتمر.

It is a very common MSA pattern meaning both ... and ....

So:

كل من المشارك والمتحدث
= both the participant and the speaker

Literally, it is closer to each of the participant and the speaker, but in natural English both is usually the best translation.

That is a very common question. In this sentence, من is not being used with the basic meaning from.

In the expression كل من X وY, من is part of a fixed structure. You can think of it as helping form the meaning each of or both.

So here:

كل من المشارك والمتحدث
does not mean all from the participant and the speaker or anything like that.

It simply means:

both the participant and the speaker

Because in the structure كل من X وY, Arabic normally names each person or thing individually in the singular.

So:

• المشارك = the participant
• المتحدث = the speaker

Then و joins them, and the whole phrase is understood as referring to both of them together.

This is different from English, where learners might expect something more like a dual form. Arabic does not usually do that here.

Because they are definite:

• المشارك = the participant
• المتحدث = the speaker

The sentence is talking about specific people, not just any participant or any speaker.

If you removed الـ, the meaning would become more general:

• مشارك = a participant
• متحدث = a speaker

So the definite article here matches the English idea of the participant and the speaker.

Because Arabic often omits the present-tense verb to be in nominal sentences.

In English, you say:

Both the participant and the speaker are at the conference headquarters.

In Arabic, the equivalent present-tense are is usually not said:

كل من المشارك والمتحدث في مقر المؤتمر.

This is completely normal Arabic.

If you wanted the past tense, then Arabic would use a verb, for example:

كان كل من المشارك والمتحدث في مقر المؤتمر
= Both the participant and the speaker were at the conference headquarters.

This is a nominal sentence.

It has two main parts:

1. كل من المشارك والمتحدث

   • the topic / subject part

2. في مقر المؤتمر

   • the predicate / comment part

The second part is a prepositional phrase, and in Arabic a prepositional phrase can serve as the predicate of a nominal sentence.

So the structure is basically:

[Both the participant and the speaker] [at the conference headquarters].

With the meaning understood as:

Both the participant and the speaker are at the conference headquarters.

It means at the conference headquarters, at the conference venue, or on the conference premises, depending on context.

Breakdown:

• في = in / at
• مقر = headquarters, seat, premises, venue
• المؤتمر = the conference

So:

في مقر المؤتمر
= at the conference headquarters / venue

The exact English wording depends on context, because مقر can be translated in a few different ways.

Because this is an iḍāfa construction, often called a possession or noun-linking structure.

In مقر المؤتمر:

• مقر = the first noun
• المؤتمر = the second noun

Together they mean something like:

• the conference headquarters
• the headquarters of the conference

In an iḍāfa, the first noun usually does not take الـ. Its definiteness comes from the second noun if the second noun is definite.

So:

• مقر مؤتمر = a conference headquarters / a conference venue
• مقر المؤتمر = the conference headquarters / venue

That is why المقر المؤتمر would be incorrect here.

A fully vowelled version would be:

كُلٌّ مِنَ الْمُشَارِكِ وَالْمُتَحَدِّثِ فِي مَقَرِّ الْمُؤْتَمَرِ

Here is what is happening:

• كُلٌّ is nominative, because it is the subject/topic of the sentence.
• المشاركِ is genitive because it comes after من.
• المتحدثِ is also genitive for the same reason.
• مقرِّ is genitive because it comes after في.
• المؤتمرِ is genitive because it is the second term of the iḍāfa.

In normal Arabic writing, these short vowels are usually not written, so learners have to infer them from the grammar.

Yes, you could.

المشارك والمتحدث في مقر المؤتمر
also means:

The participant and the speaker are at the conference headquarters.

Adding كل من makes the meaning both the participant and the speaker more explicit. It can also sound a bit more formal or written.

So the difference is roughly:

• المشارك والمتحدث ...
  = the participant and the speaker ...

• كل من المشارك والمتحدث ...
  = both the participant and the speaker ...

Both are correct, but كل من adds emphasis to the idea of including each member of the pair.