guruupu no messeezi ni sutanpu dake okutte, saki ni sigoto ni modotta.

の links two nouns. Here, グループのメッセージ means something like the group’s message, a message in the group, or more naturally in English, the group message / group chat message.

It shows that メッセージ is related to グループ. In casual real-life translation, this often just means the group chat or the group thread.

So:

• グループ = group
• メッセージ = message
• グループのメッセージ = the group message / message in the group chat

The に marks the target or destination of 送る (to send).

So メッセージにスタンプを送る means to send a stamp/sticker to the message/chat. In natural English, we would usually say send a sticker in the group chat or send a sticker to the group chat.

This に works a lot like to in English:

• 友だちに送る = send to a friend
• グループのメッセージに送る = send to the group message/chat

だけ means only or just.

So スタンプだけ送って means sending only a sticker or just sent a sticker.

It implies that the speaker did not type a full reply. They only reacted with a sticker/stamp.

Examples:

• スタンプだけ送った = I sent only a sticker
• 一言だけ言った = I said only one thing
• 水だけ飲んだ = I drank only water

送って is the て-form of 送る. The て-form is often used to connect actions.

So:

• スタンプだけ送って、先に仕事に戻った。 means
• I sent only a sticker, and then went back to work.

The て-form here links the first action to the second action in sequence.

This is very common in Japanese:

• 朝ごはんを食べて、出かけた。 = I ate breakfast and went out.
• メールを読んで、返事した。 = I read the email and replied.

Only the final verb, 戻った, carries the past tense for the whole sequence.

先に can mean first, before that, ahead, or earlier than others, depending on context.

In this sentence, it most likely means something like:

• I went back to work first
• I went back to work before doing anything else
• I returned to work ahead of the others / before continuing the conversation

It gives the feeling that the speaker did a quick action in the chat, then returned to work right away.

So 先に adds nuance, not just basic sequence. It suggests going on ahead or resuming work early/first.

With 戻る (to return), the place/state you return to is often marked with に.

So:

• 仕事に戻る = return to work
• 家に戻る = return home
• 席に戻る = return to one’s seat

Here, 仕事 is treated as the thing the speaker is returning to, so 仕事に戻った means returned to work.

The subject is omitted, which is very normal in Japanese.

From context, the natural interpretation is I:

• (I) sent only a sticker in the group chat and went back to work.

Japanese often leaves out subjects when they are obvious from context. So unless the wider conversation suggests someone else, learners should usually understand this as I or the speaker.

In modern Japanese, スタンプ often refers to a digital sticker, especially in messaging apps like LINE.

So in this sentence, スタンプ does not mean a postage stamp. It means a visual reaction image or sticker sent in chat.

This is a common usage in everyday Japanese:

• スタンプを送る = send a sticker
• スタンプだけで返事する = reply using only a sticker

It is fairly casual/plain.

Clues:

• 戻った is plain past, not polite 戻りました
• the sentence simply connects actions with て
• there is no explicit polite phrasing

A more polite version could be:

• グループのメッセージにスタンプだけ送って、先に仕事に戻りました。

So the original sounds natural in conversation, texting, narration, or informal writing.

Yes, it can be understood, but depending on context, many speakers might also say:

• グループチャット
• グループLINE
• グループのトーク

グループのメッセージ is understandable as the group message/thread/chat, but it may sound a little more descriptive than the most everyday app-specific wording.

So it is not wrong, but in real conversation the exact expression may vary depending on the app and situation.