Andrei se alătură grupului.

Because a se alătura is the usual Romanian verb meaning to join in the sense of to join oneself to a group, cause, team, etc.

So:

• a se alătura = to join
• se is the reflexive marker used with this verb

It does not mean that Andrei is literally doing something to himself in English. It is just part of how this verb is normally used in Romanian.

Alătură is present tense, 3rd person singular.

So:

• eu mă alătur = I join / I am joining
• tu te alături = you join
• el/ea se alătură = he/she joins
• Andrei se alătură = Andrei joins

In this sentence, the verb agrees with Andrei, which is a singular subject.

Because a se alătura takes an indirect object in Romanian, usually in the dative case.

So:

• grupul = the group (subject/direct-object form)
• grupului = to the group (dative/genitive form)

In English, we say join the group, but Romanian structures it more like join oneself to the group.

That is why grupului is correct here.

In grupului, -ului shows that the noun is:

• masculine/neuter singular
• definite = the group
• in the dative/genitive form

So grupului means to the group or of the group, depending on context.

Here it means to the group because the verb a se alătura requires the dative.

Yes. Andrei is the subject, and se alătură is the verb phrase.

So the sentence breaks down like this:

• Andrei = subject
• se alătură = joins
• grupului = to the group

Even though English says joins the group, Romanian uses the dative for grupului.

Yes, Romanian word order is fairly flexible, though the neutral order is:

• Andrei se alătură grupului.

You may also hear other orders for emphasis, such as:

• Grupului i se alătură Andrei. = Andrei joins the group / It is Andrei who joins the group
• Se alătură grupului Andrei. = more literary or context-dependent

For a learner, the standard version Andrei se alătură grupului is the safest and most natural starting point.

Not with the meaning Andrei joins the group.

Without se, the verb changes meaning and structure. A alătura can mean something like to place next to, to attach, or to put alongside something.

So:

• a se alătura = to join
• a alătura = to put together / place alongside / attach

That is why se is necessary in this sentence.

A rough pronunciation guide:

• alătură ≈ uh-luh-TOO-ruh
• grupului ≈ GROO-poo-looy

A few helpful notes:

• ă is a short neutral vowel, like the a in about
• u is like oo in food
• ei in Andrei sounds roughly like ay

So Andrei se alătură grupului is roughly:

ahn-DRAY se uh-luh-TOO-ruh GROO-poo-looy

Because this verb normally links directly to a noun in the dative.

English often uses one structure, while Romanian uses another. With a se alătura, Romanian prefers:

• se alătură grupului = joins the group

not usually:

• se alătură la grup

So even though English learners may expect a preposition, Romanian expresses this relationship through the noun form grupului.

The present tense in Romanian can do both, just like English in some contexts.

So Andrei se alătură grupului can mean:

• Andrei joins the group
  or, depending on context,
• Andrei is joining the group

The wider context tells you which meaning is intended.

Grup is a neuter noun in Romanian.

That means:

• in the singular, it behaves like a masculine noun
• in the plural, it behaves like a feminine noun

So:

• singular definite nominative/accusative: grupul
• singular dative/genitive: grupului
• plural indefinite: grupuri
• plural definite: grupurile

In this sentence, the singular dative form is used: grupului.

Yes, depending on context.

For example:

• a se alătura = to join, to align oneself with
• a intra în = to enter / join in some contexts
  
  • Andrei intră în grup = Andrei enters the group
• a adera la = to adhere to / join formally
  
  • Andrei aderă la organizație = Andrei joins the organization

But for the sentence you were given, a se alătura grupului is a very natural and standard way to say Andrei joins the group.