Okupićemo se u stanu.

It is the 1st person plural future tense of the verb okupiti se.

• okupiti se = to gather / to get together
• okupićemo se = we will gather / we will get together

So the ending tells you the subject is we, even though mi is not written.

This is a normal way to form the future tense in Serbian.

The future auxiliary comes from hteti (to want) in its short form:

• ću
• ćeš
• će
• ćemo
• ćete
• će

With many verbs, this short future form attaches to the infinitive stem:

• okupiti
  • ćemo → okupićemo

Notice that the final -i of okupiti drops before ćemo attaches.

A very common alternative is:

• Mi ćemo se okupiti.

That means the same thing.

Se is the reflexive particle.

Here it shows that the verb is okupiti se, not just okupiti.

Compare:

• okupiti nekoga = to gather someone / bring people together
• okupiti se = to gather, to get together

So in this sentence, se is necessary because the meaning is we will gather/get together, not we will gather something or someone.

Because Serbian usually does not need subject pronouns when the verb ending already shows who the subject is.

Here, -mo in okupićemo tells you the subject is we.

So:

• Okupićemo se u stanu. = normal, natural
• Mi ćemo se okupiti u stanu. = also correct, but mi adds emphasis or contrast

For example, mi might be used if you want to stress we, not someone else.

Because Serbian uses different cases after u depending on whether you mean:

• location → u + locative
• movement into something → u + accusative

Here the idea is where the gathering will happen: in the apartment.
So you need the locative:

• u stanu = in the apartment

If you were talking about motion into the apartment, you would use:

• u stan = into the apartment

Because stan is in the locative singular after u.

The dictionary form is:

• stan = nominative singular

But after u for location, it becomes:

• u stanu

This is a normal pattern for many masculine nouns.

It is perfective.

That means it presents the action as a complete event: a gathering that will happen as a whole.

That fits this sentence well, because it refers to one future event.

The imperfective partner is okupljati se, which is more like:

• gathering repeatedly
• gathering over time
• a habitual action

For example:

• Okupljaćemo se u stanu svakog petka. = We will gather in the apartment every Friday.

So okupićemo se is the natural choice for a single planned get-together.

Yes. Serbian word order is fairly flexible.

The sentence:

• Okupićemo se u stanu.

is a neutral, natural way to say it.

But you can also say:

• U stanu ćemo se okupiti.
  This puts more emphasis on where.
• Mi ćemo se okupiti u stanu.
  This emphasizes we.

The meaning stays basically the same, but the focus shifts.

Yes, absolutely. That is fully correct and very common.

It is the same future tense, just built in the separate way:

• Mi ćemo se okupiti u stanu.

instead of the attached form:

• Okupićemo se u stanu.

Both are standard Serbian.

A useful note: in standard Serbian, okupiti ćemo se is generally not the preferred form.
You normally say either:

• okupićemo se or
• mi ćemo se okupiti

Ć is a soft consonant that English speakers often find tricky.

A rough approximation is something like a very soft ty/ch sound. It is softer than č.

So:

• ć = softer
• č = harder, more like the ch in church

A rough learner-friendly pronunciation of okupićemo could be:

• oh-koo-PEE-tyeh-mo

That is only approximate, but it helps.

Literally, it means in the apartment.

Depending on context, English might translate it more naturally as:

• We’ll get together in the apartment.
• We’ll meet at the apartment.
• We’ll gather at the apartment.

So the Serbian is literal and straightforward, but English may choose a slightly different natural phrasing.

It is neutral. It works in everyday speech and in normal written language.

Nothing in the sentence is especially formal, slangy, or unusual. It is a standard, natural Serbian sentence.