Она решила присоединиться к группе.

Because the subject is она (she), and in the Russian past tense, the verb agrees with the subject in gender and number.

• решил = he decided
• решила = she decided
• решило = it decided
• решили = they decided

So Она решила means she decided.

Russian past tense is usually formed from the verb stem plus a form of -л:

• masculine: -л
• feminine: -ла
• neuter: -ло
• plural: -ли

So from решить (to decide), you get:

• решил
• решила
• решило
• решили

In this sentence, решила is feminine singular because the subject is она.

Because after решила (decided), Russian normally uses an infinitive to say what someone decided to do.

So the structure is:

• она решила + infinitive
• she decided + to + verb

Examples:

• Она решила уйти. = She decided to leave.
• Она решила позвонить. = She decided to call.
• Она решила присоединиться к группе. = She decided to join the group.

This is a question of aspect.

• присоединяться = imperfective
• присоединиться = perfective

Here, присоединиться is used because the sentence refers to a single completed action: the decision was to join the group, as one whole event.

Very roughly:

• присоединяться = to be joining / to join habitually / in process
• присоединиться = to join successfully, as a complete action

After решить, Russian often uses a perfective infinitive when talking about deciding to carry out a one-time action.

The -ся ending marks the verb as reflexive, but in many verbs it does not literally mean oneself in the English sense. It is simply part of the verb’s normal form.

So присоединиться is the standard verb meaning to join, to attach oneself to, or to become part of something.

You should learn it as a whole verb:

• присоединиться к чему-либо = to join something / to join in with something

In this sentence, it does not need to be translated word-for-word as join oneself. Just think of it as the normal Russian verb for join in this pattern.

Because присоединиться requires the preposition к plus the dative case.

The pattern is:

• присоединиться к + dative

So:

• к группе = to the group

This is different from English, where we simply say join the group without a preposition. Russian expresses the idea more like attach oneself to the group.

More examples:

• присоединиться к команде = to join the team
• присоединиться к разговору = to join the conversation
• присоединиться к нам = to join us

Because after к, the noun must be in the dative case.

The noun группа is feminine and ends in -а. For many feminine nouns of this type:

• nominative: группа
• dative: группе

So:

• группа = group
• к группе = to the group

This is a very common pattern:

• мама → к маме
• школа → к школе
• комната → к комнате

Yes. Russian word order is more flexible than English because the grammar is shown by endings, not only by position.

The neutral order here is:

• Она решила присоединиться к группе.

But other orders are possible, depending on emphasis:

• К группе она решила присоединиться.
  This emphasizes to the group.
• Присоединиться к группе она решила.
  This emphasizes the action or may sound more literary.

For a learner, the original sentence is the most natural neutral version.

Sometimes yes, if the subject is already clear from context.

Russian often drops subject pronouns when they are understood:

• Решила присоединиться к группе. = [She] decided to join the group.

However, она is kept when:

• you want to make the subject explicit,
• you are introducing a new subject,
• or you want contrast or emphasis.

So both can work, but Она решила присоединиться к группе is perfectly natural.

Yes, there can be a small difference.

• присоединиться к группе often means to join up with a group, to join in with a group, or to become part of a group
• вступить в группу can sound more like to enter or become a member of a group, sometimes a more formal organization

So in many contexts they may both translate as join the group, but присоединиться к группе often focuses on coming together with the group rather than formal membership.

The main stresses are:

• Она́
• реши́ла
• присоедини́ться
• к гру́ппе

A helpful pronunciation guide:

• Она́ = a-NA
• реши́ла = ree-SHEE-la
• присоедини́ться = pree-sa-ee-dee-NEETS-a
• к гру́ппе = k GROO-pee

Stress is very important in Russian because it is not always predictable, so it is good to learn each word together with its stress.