Learn Hardcore Croatian: Na sastanku svi sudjeluju bez stresa. - At the meeting everyone participates without stress.

Sastanku is in the locative singular case of the noun sastanak (meeting).
Masculine nouns ending in -ak often change to -ku in the locative: sastanak → (o) sastanku, vlak → (na) vlaku, etc.
The locative is almost always used with certain prepositions, such as na, u, o, po. Here, na triggers the locative, so na sastanku must be locative, not dative.

Why is the preposition na used with sastanku instead of u?

In Croatian, na is typically used for events, activities, and gatherings, like na sastanku (at the meeting), na koncertu (at the concert), na predavanju (at the lecture).
U is more about being inside a physical space: u sobi (in the room), u zgradi (in the building).
So na sastanku is the natural way to say at the meeting, even though in English we say at rather than on.

Why is there no word for the before sastanku?

Croatian has no articles (no direct equivalents of a/an/the).
A noun like sastanak/sastanku can mean a meeting, the meeting, or meetings in general, depending on context.
Here, Na sastanku is understood from context as At the meeting, but the grammar itself does not mark definiteness.

What is the infinitive and conjugation of sudjeluju?

The infinitive is sudjelovati (to participate, to take part).
Sudjeluju is 3rd person plural, present tense: oni/one/ona sudjeluju (they participate).
Present tense of sudjelovati:

ja sudjelujem
ti sudjeluješ
on/ona/ono sudjeluje
mi sudjelujemo
vi sudjelujete
oni/one/ona sudjeluju

Does sudjelovati need a preposition like u or na after it?

Yes, when you say what you participate in, sudjelovati is normally followed by u or na + locative:

sudjelovati u projektu – to participate in a project
sudjelovati na sastanku – to participate in a meeting
In the sentence Na sastanku svi sudjeluju bez stresa, the phrase na sastanku is moved to the beginning for emphasis, but it still logically belongs with sudjeluju.
So the underlying structure is basically Svi sudjeluju na sastanku bez stresa.

Why is it svi sudjeluju and not svatko sudjeluje?

Both are possible, but they have a slightly different feel.
Svi sudjeluju literally means everyone (all of them) participate, describing the group as a whole.
Svatko sudjeluje is more like each person participates, focusing on individuals one by one.
Grammatically, svi takes a plural verb (sudjeluju), while svatko takes a singular verb (sudjeluje).

Why is it bez stresa and not something like bez stres or bez stresu?

The preposition bez (without) always takes the genitive case.
The noun stres (stress) in the genitive singular is stresa, so bez stres‑a is the only correct form.
This pattern is regular: bez šećera (without sugar), bez novca (without money), bez problema (without problems).

Can I say bez ikakvog stresa or bez stresa uopće instead of just bez stresa?

Yes, both are correct, but they add emphasis.

bez ikakvog stresa = without any stress at all
bez stresa uopće = without stress at all / not stressed at all
Plain bez stresa is neutral and already means without stress, but these longer versions sound stronger and more emphatic.

Can I change the word order? Is Svi na sastanku sudjeluju bez stresa also correct?

Yes, Croatian word order is quite flexible, and Svi na sastanku sudjeluju bez stresa is grammatically correct.
The original Na sastanku svi sudjeluju bez stresa puts more emphasis on Na sastanku (at the meeting) as the setting.
Starting with Svi puts more focus on the participants: Everyone, at the meeting, participates without stress.
The basic meaning stays the same; the nuance of what is highlighted changes slightly.

Does this sentence refer to one particular meeting, or to meetings in general?

By itself, Na sastanku svi sudjeluju bez stresa most naturally sounds like one specific meeting that the speaker has in mind.
If you want to clearly talk about meetings in general, you would usually use the plural:
Na sastancima svi sudjeluju bez stresa. – At meetings, everyone participates without stress.
Context often decides whether the singular is meant as a specific meeting or as a type of situation.

How would the sentence change if I were talking about just one person instead of everyone?

You would switch from svi (everyone) to a singular subject and use the singular verb form sudjeluje. For example:

Na sastanku on sudjeluje bez stresa. – At the meeting, he participates without stress.
Na sastanku ona sudjeluje bez stresa. – At the meeting, she participates without stress.
If the subject is understood from context, you can even drop the pronoun: Na sastanku sudjeluje bez stresa.

Are there any useful synonyms for sudjelovati that I should know?

Yes, the closest everyday synonym is učestvovati, which also means to participate and is used in very similar contexts.
Another one is participirati, which sounds more formal/technical and is often used in academic, legal, or bureaucratic language.
So you might hear: sudjelovati u projektu, učestvovati u projektu, or participirati u projektu, with only small differences in style and register.