В конце ноября мы с друзьями уже обсуждаем планы на декабрь.

В конце ноября is a fixed-type time expression meaning at the end of November or in late November.

• в конце: конце is the prepositional singular of конец.
• ноября: this is the genitive singular of ноябрь.

Why those cases?

• в
  • prepositional is often used for location or time within something.
• конец чего? takes the genitive, so ноября means of November.

So literally, the phrase is something like in the end of November, but in natural English we say at the end of November.

Because с meaning with normally takes the instrumental case when it means accompaniment.

So:

• друзья = friends
• с друзьями = with friends

This is one of the most common uses of the instrumental in Russian:

• с братом = with my brother
• с мамой = with mom
• с друзьями = with friends

In this sentence, мы с друзьями is most naturally understood as my friends and I or we, together with our friends, depending on context.

Literally, it looks like we with friends, but idiomatically Russian often uses this structure to name the group involved.

So a natural English translation would usually be:

• My friends and I are already discussing plans for December at the end of November.

The important thing grammatically is that the main subject is мы, so the verb is in the 1st person plural: обсуждаем.

Because the sentence is describing what happens at that time: by late November, we are already discussing December plans.

Russian uses the present tense here just like English can:

• In late November, we’re already discussing plans for December.

It can sound like:

• a description of what is happening now,
• or a general/habitual fact about what people do every year.

So the present tense is completely natural.

Обсуждаем is imperfective, and that fits the idea of an ongoing process: discussing, talking over, going through the plans.

The imperfective is used when the focus is on:

• the process,
• the activity itself,
• repetition or duration.

A perfective form like обсудим would mean something more like we will discuss and finish discussing.

Compare:

• обсуждаем планы = we are discussing plans
• обсудим планы = we will discuss the plans / we’ll go over the plans

In this sentence, the point is not completion, but the fact that the discussion is already happening.

Планы на декабрь means plans for December.

In Russian, план на + accusative is very common when talking about something intended, scheduled, or meant for a certain time period or purpose.

So:

• планы на декабрь = plans for December
• билеты на концерт = tickets for the concert
• деньги на отпуск = money for vacation

Here декабрь becomes декабрь in the accusative, but since it is an inanimate masculine noun, the accusative looks the same as the nominative:

• декабрь → на декабрь

Уже means already.

It adds the idea that this is happening earlier than one might expect or that the process has started by this point.

So the sentence is not just:

• At the end of November, we discuss plans for December

but rather:

• At the end of November, we’re already discussing plans for December

That already gives a sense like:

• Wow, it’s not even December yet, and we’ve started talking about it.

Yes. Russian word order is more flexible than English word order.

The sentence:

В конце ноября мы с друзьями уже обсуждаем планы на декабрь.

is very natural because it starts with the time expression, setting the scene first:

• At the end of November...

Other word orders are possible, for example:

• Мы с друзьями уже обсуждаем планы на декабрь в конце ноября.
• Мы уже в конце ноября обсуждаем планы на декабрь.

But the original version is smooth and neutral. It puts emphasis first on when this happens.

Russian word order often changes not the basic meaning, but the focus or information structure.

No, they are close in meaning but not the same.

• в конце ноября = at the end of November / in late November
• к концу ноября = by the end of November

So:

• В конце ноября мы обсуждаем планы...
  = the discussion happens in late November

• К концу ноября мы уже обсудили планы...
  = by the end of November, we have already discussed the plans

In your sentence, в конце ноября is correct because it places the action during that time period, not by a deadline.