Húsfélagið hittist í anddyrinu í kvöld.

Húsfélagið is a compound noun:

• hús = house / building
• félag = association / society / group

Together, húsfélag usually means something like a building association, residents’ association, or condo/homeowners’ association, depending on context.

The -ið at the end is the definite article, attached to the noun itself, so:

• húsfélag = a building association
• húsfélagið = the building association / the residents’ association

It is a neuter singular noun.

Because the verb here is hittast, not hitta.

• hitta = to find / to meet someone
• hittast = to meet each other / to get together

The -st form often gives a middle/reflexive/reciprocal meaning in Icelandic. In this sentence, the idea is that the members of the association are meeting one another, so hittast is the natural verb.

So:

• hittir would come from hitta
• hittist is the present singular form of hittast

It is present tense.

Even though English often says is meeting tonight, Icelandic very commonly uses the simple present for planned or scheduled future events:

• Húsfélagið hittist í kvöld.
  = The association meets tonight / is meeting tonight.

So the tense is present, but the time expression í kvöld makes it clear that the event is happening later today.

The basic noun is anddyri, which means entrance hall / foyer / lobby.

Here, anddyrinu is:

• singular
• definite = the lobby
• dative

Why dative? Because the preposition í can take different cases:

• accusative for motion into something
• dative for location in something

So:

• í anddyri / í anddyrið would suggest movement into the lobby
• í anddyrinu means being located in the lobby

In this sentence, the meeting takes place there, so it is location, and the dative is used.

Because the sentence has two different prepositional expressions:

• í anddyrinu = in the lobby
• í kvöld = tonight

The first one gives the place. The second one gives the time.

This is completely normal in Icelandic.

Icelandic usually uses í for being inside an enclosed space.

Since a lobby or entrance hall is an interior space, í is the natural choice:

• í anddyrinu = in the lobby

By contrast, á is more often used for meanings like on, or for certain locations where Icelandic idiom prefers á. But for a room-like indoor space, í is what you would expect.

Because í kvöld is a fixed, very common time expression meaning tonight.

Icelandic often uses short prepositional time phrases like this without the definite article:

• í dag = today
• í kvöld = tonight
• í nótt = tonight / during the night

So í kvöld is just the normal idiomatic way to say it.

Yes. Grammatically, húsfélagið is singular, because it refers to the association as one unit.

That is why the verb is singular too:

• Húsfélagið hittist...

English does the same kind of thing with words like the committee, the team, or the association.

If you wanted to focus on the individual people instead, Icelandic might use a plural noun such as:

• íbúarnir = the residents
• húsfélagsmennirnir = the association members

Yes. Icelandic word order is flexible, but it still follows important rules, especially the verb-second pattern in main clauses.

The original sentence is:

• Húsfélagið hittist í anddyrinu í kvöld.

You could also say:

• Í kvöld hittist húsfélagið í anddyrinu.

That puts more emphasis on tonight. Notice that when Í kvöld moves to the front, the verb hittist still stays in second position.

So Icelandic allows reordering for emphasis, but the verb placement is still tightly controlled.

Then Icelandic would normally show direction/movement instead of location.

With í, that often means using the accusative rather than the dative.

So the contrast is roughly:

• í anddyrinu = in the lobby, located there
• í anddyrið = into the lobby

In your sentence, the association is meeting there, not moving there, so í anddyrinu is the correct form.