De gastenlijst staat in de groepschat.

Because gastenlijst is a de-word in Dutch.

Dutch nouns take either de or het as their definite article:

• de tafel
• het huis
• de gastenlijst

Unfortunately, whether a noun takes de or het often has to be learned word by word. Here, the correct form is de gastenlijst.

A useful detail:

• In the plural, Dutch nouns always take de, but here gastenlijst is singular. It is just one of the nouns that naturally takes de.

Gastenlijst is a compound noun:

• gast = guest
• gasten = guests
• lijst = list

So gastenlijst literally means guest list or list of guests.

Dutch very often combines nouns into a single word where English uses two separate words. So:

• English: guest list
• Dutch: gastenlijst

The -en in the middle comes from the plural form gasten and is very common in Dutch compounds.

Dutch often uses staan where English would simply use to be, especially when talking about written information, text, or something shown somewhere.

So:

• De gastenlijst staat in de groepschat. = The guest list is in the group chat.

But more literally, Dutch is expressing something like:

• The guest list stands/appears in the group chat.

This is very natural in Dutch when something is:

• written somewhere
• posted somewhere
• printed somewhere
• listed somewhere

For example:

• Het adres staat op de website. = The address is on the website.
• De tijd staat in de e-mail. = The time is in the email.

So staat is not strange here; it is the normal Dutch choice.

Staat is the third person singular present tense of staan.

The verb staan means to stand, but it is also used in broader ways in Dutch.

Present tense:

• ik sta = I stand / I am standing
• jij staat = you stand
• hij/zij/het staat = he/she/it stands
• de gastenlijst staat = the guest list stands / is

Because de gastenlijst is singular, the verb must also be singular:

• De gastenlijst staat

If the subject were plural, you would use staan:

• De gastenlijsten staan in de groepschat. = The guest lists are in the group chat.

This is normal Dutch main-clause word order:

subject + verb + rest

So here:

• De gastenlijst = subject
• staat = verb
• in de groepschat = prepositional phrase

Dutch main clauses usually put the conjugated verb in the second position:

• De gastenlijst staat in de groepschat.

You can move other elements to the front, but then the verb still stays in second position:

• In de groepschat staat de gastenlijst.

That version is also grammatical, but the original sentence is the most neutral and natural.

In Dutch, in is normally used for something being inside a chat, message thread, document, book, email, and so on.

So:

• in de groepschat = in the group chat

Dutch often uses:

• in for something contained within a conversation, app thread, document, or text space
• op more for surfaces or some platforms/websites in certain contexts

Examples:

• in de chat
• in de e-mail
• in het document

So in de groepschat is the natural choice here.

Groepschat is another compound noun:

• groep = group
• chat = chat

Together:

• groepschat = group chat

Again, Dutch writes this as one word, while English uses two words.

A few similar examples:

• groepsapp = group app chat / group WhatsApp chat
• klasgenoot = classmate
• telefoonnummer = phone number

So if you are used to English, one very common adjustment in Dutch is learning to recognize these long compound words as normal single nouns.

That -s- is a common linking sound in Dutch compounds.

You start with:

• groep = group
• chat = chat

But in the compound, Dutch uses:

• groepschat

This linking -s- does not always have a simple rule that learners can predict perfectly, so it is often best to learn common compounds as complete words.

Other examples with a linking -s:

• dorpsplein = village square
• levensverhaal = life story
• liefdesbrief = love letter

So groepschat is just the standard form.

Yes, people would probably understand you, and in some situations it may sound acceptable. But staat is more idiomatic and more natural if you mean the guest list is written or posted there.

Compare:

• De gastenlijst staat in de groepschat.
  This sounds natural for posted information.
• De gastenlijst is in de groepschat.
  Understandable, but less typically Dutch in this context.

So if you want to sound natural, staat is the better choice here.

It is singular.

Even though gasten inside the compound is plural, the whole noun gastenlijst refers to one list.

That is why the verb is singular:

• De gastenlijst staat ...

If it were plural, it would be:

• De gastenlijsten staan in de groepschat. = The guest lists are in the group chat.

This is a helpful thing to remember with compounds: the whole word behaves grammatically as one noun.

A rough pronunciation guide is:

duh GHAS-tun-lijst staht in duh khroops-khat

A few useful pronunciation notes:

• de is usually pronounced like a weak duh
• g in standard Dutch is a throaty sound, not like English g
• ui is not in this sentence, so that helps
• ij in lijst sounds somewhat like the vowel in English eye
• ch in chat is usually like the English ch, because chat is a borrowed word

Word by word:

• De → duh
• gastenlijst → roughly GHAS-tun-lijst
• staat → roughly staht
• in → in
• de → duh
• groepschat → roughly khroops-khat

The exact pronunciation depends somewhat on accent and region, but this should help you recognize and say it.