Learn Hardcore Japanese: tomodati to issyo ni supootu wo suru to, motto tanosimeru. - When I play sports together with friends, I can enjoy it more.

That’s still natural.
Using both と and 一緒に just emphasizes the idea of doing it “together with” someone. It sounds a bit more explicit/warmer than 友達と alone.

What is the role of に after 一緒? Why 一緒に and not just 一緒?

一緒に is an adverbial phrase meaning “together”.

一緒 by itself is a noun-ish word meaning “together / the same,” but to use it like “do X together,” it normally takes に and becomes 一緒に, modifying the verb:

Examples:

一緒に行きましょう。 – Let’s go together.
一緒に勉強する。 – Study together.

So:

友達と一緒にスポーツをする
= (do sports) together (に) with (と) friends (友達)

Could we say 友達とスポーツをする without 一緒に? Does the meaning change?

Yes, you can say:

友達とスポーツをする – play sports with friends

The meaning is very close. 一緒に adds a bit of explicit “togetherness,” but in most contexts, 友達とスポーツをする already implies “together with friends,” so it’s perfectly natural.

Nuance:

友達とスポーツをする – simple “I play sports with friends.”
友達と一緒にスポーツをする – slightly more emphasis on “doing it together” (can feel a little more friendly/explicit).

Why is it スポーツをする instead of just スポーツする? Is を necessary?

Both are possible in casual Japanese:

スポーツをする
スポーツする

Grammatically, スポーツをする is more “complete”:

スポーツ (sport) + を (object marker) + する (to do)

In spoken, especially casual, Japanese, the を is often dropped after する:

勉強をする → 勉強する
掃除をする → 掃除する
スポーツをする → スポーツする

Your sentence uses the fuller form スポーツをする, which is totally standard and maybe a bit clearer/“textbooky.” In many real-life contexts, you’d also hear スポーツする.

What does と after する do here? Is it a quotation marker, or does it mean “if/when”?

The と after する here is the conditional と, meaning “when / if / whenever.”

So:

スポーツをすると、もっと楽しめる。
≈ “When you play sports, you can enjoy it more.” / “If you play sports, you can enjoy it more.”

This と is not the same as the と for quotations like:

「行こう」と言った。 – He said, “Let’s go.”

Here it links a condition (スポーツをする) with a result (もっと楽しめる).

What’s the difference between this conditional と and たら (or とき)?

Very briefly:

Vると …
Often implies a natural result, something that always or generally happens when that condition is met.
- 春になると暖かくなる。 – When it becomes spring, it gets warm.
Vたら …
More like a general “if/when (one time)”; it can be used for more personal, one-off situations.
- 時間があったら、映画を見よう。 – If/when we have time, let’s watch a movie.
Vるとき / Vたとき
Means “when (at the time that) …”, focusing on the time point, not necessarily a cause–effect relationship.
- 学校に行くとき、バスに乗る。 – When I go to school, I take the bus.

In your sentence:

友達と一緒にスポーツをすると、もっと楽しめる。

The と suggests a regular, typical result: “Whenever/when you play sports with friends, you can enjoy it more (than otherwise).”
たら would also be possible, but と nicely fits the idea of a general truth.

Why is it 楽しめる and not just 楽しい or 楽しむ?

楽しめる is the potential form of 楽しむ (“to enjoy”), meaning “can enjoy / be able to enjoy.”

楽しむ – to enjoy (actively)
楽しめる – to be able to enjoy / can enjoy

So:

もっと楽しめる。
literally: “(You) can enjoy (it) more.”

If you said:

もっと楽しい。 – “It’s more fun.”

That describes the thing as more fun.
楽しめる focuses on your ability/experience of enjoyment.

So the sentence is emphasizing the experience:
“When you play sports with friends, you’re able to enjoy it more.”

What exactly is もっと modifying? Is it “more fun,” “enjoy more,” or something else?

もっと means “more” and here it modifies 楽しめる:

もっと楽しめる = “can enjoy (it) more”

So the implied meaning is “you can enjoy it more (than if you didn’t play with friends)” → i.e. “it’s more enjoyable / more fun for you.”

Grammatically:

もっと (more) + 楽しめる (can enjoy)
≈ “can enjoy more”

Is 楽しめる polite or casual? How would I say this politely?

楽しめる is plain (dictionary) form, so the sentence is casual / neutral plain style.

To make it polite, change the verb ending to ます form:

友達と一緒にスポーツをすると、もっと楽しめます。

Changes:

楽しめる → 楽しめます

Everything else can stay the same.
Polite version: “…と、もっと楽しめます。”

Does 友達 mean “friend” or “friends” here? There’s no plural -s.

In Japanese, 友達 doesn’t mark singular vs plural by default. It can mean:

“a friend”
“friends”
“my friend(s)” / “your friend(s)” depending on context

In a general statement like:

友達と一緒にスポーツをすると、もっと楽しめる。

English will usually translate this as “friends”:

“When you play sports with friends, you can enjoy it more.”

So you should understand 友達 here as “friend(s)” in a general sense, not specifically one friend or many.

Is there anything special about the comma 、 after すると? Could it be omitted?

The 、 after すると is mostly for readability and natural pacing:

友達と一緒にスポーツをすると、もっと楽しめる。

You can omit it in casual writing:

友達と一緒にスポーツをするともっと楽しめる。

But in standard written Japanese, especially when a subordinate clause (スポーツをすると) ends and the main clause begins (もっと楽しめる), it’s common to insert 、.

So it’s not a grammar requirement, but it’s stylistically normal and helps the reader parse the sentence.