المشارك عند مكتب التسجيل.

In Arabic, the present-tense verb to be is usually not expressed.

So المشارك عند مكتب التسجيل literally looks like the participant at the registration desk/office, but it means:

• The participant is at the registration desk.

This is very common in Arabic nominal sentences.
If you wanted the past tense, you would use كان:

• كان المشارك عند مكتب التسجيل = The participant was at the registration desk.

المشارك means the participant or the one participating.

It comes from the verb شارك = to participate / to take part.

The pattern here often gives an active participle, so مشارك means participant / participating person.
With الـ, it becomes المشارك = the participant.

الـ is the Arabic definite article, equivalent to the in English.

So:

• مشارك = a participant / participant
• المشارك = the participant

In this sentence, the speaker is referring to a specific participant, so المشارك is definite.

Both can relate to location, but they are not exactly the same.

• في usually means in / inside
• عند often means at / by / with / near, depending on context

In المشارك عند مكتب التسجيل, عند gives the sense of being at the registration desk/office, not necessarily physically inside something.

Compare:

• المشارك في المكتب = The participant is in the office
• المشارك عند المكتب = The participant is at the desk/office

So عند is a natural choice for at the registration desk.

مكتب التسجيل is an iḍāfa construction, often called a genitive construction or noun + noun phrase.

It literally consists of:

• مكتب = office / desk
• التسجيل = the registration

Together:

• مكتب التسجيل = the registration office or registration desk

In an iḍāfa:

1. the first noun usually does not take الـ
2. the second noun defines or specifies the first

So although مكتب has no الـ, the whole phrase can still be definite because التسجيل is definite.

Because in an iḍāfa construction, the first noun normally does not take the definite article.

So Arabic says:

• مكتب التسجيل

not:

• المكتب التسجيل ❌

The definiteness comes from the second noun:

• مكتب تسجيل = a registration desk / registration office
• مكتب التسجيل = the registration desk / office

This is a very important pattern in Arabic.

It can mean either one, depending on context.

• مكتب can mean desk, office, or bureau
• التسجيل means registration

So possible translations include:

• registration desk
• registration office
• registration counter (in some contexts)

If this is at an event, hotel, or conference, registration desk is often the most natural English translation.

Arabic often begins a nominal sentence with the topic or subject-like element.

So:

• المشارك عند مكتب التسجيل = The participant is at the registration desk

This structure puts focus on the participant.

You could also say:

• عند مكتب التسجيل المشارك

but that would be less neutral and would sound more marked or context-dependent.
For a simple statement, starting with المشارك is the most straightforward choice.

A careful pronunciation is approximately:

al-mu-shaa-rik ʿinda mak-tab at-tas-jeel

A few notes:

• المشارك = al-mushārik
• عند begins with ع, a sound that does not exist in English
• التسجيل is pronounced at-tasjīl or at-tasjīl, because ل in الـ assimilates to the ت

That is because ت is a sun letter, so:

• التسجيل is written with ال
• but pronounced more like at-tسجيل / at-tasjīl

In fully vowelled formal Arabic, it would typically be:

المشاركُ عندَ مكتبِ التسجيلِ

Here is why:

• المشاركُ: usually ḍamma because it is the subject/topic of the nominal sentence
• عندَ: often has fatḥa
• مكتبِ: majrūr after عند, so it takes kasra
• التسجيلِ: also majrūr because it is the second part of the iḍāfa

In normal everyday writing, these short vowels are usually omitted.

Yes. عند has several common uses.

It can mean:

• at
• with
• near / by
• sometimes in the possession of

For example:

• عندي كتاب = I have a book
  Literally: At me is a book

But in your sentence, the meaning is clearly location:

• المشارك عند مكتب التسجيل = The participant is at the registration desk