This example uses the emoji module to easily add emoji in your text messages and filters to make it only work for specific messages in a specific chat.

from pyrogram import Client, emoji, filters

# Target chat. Can also be a list of multiple chat ids/usernames
TARGET = -100123456789
# Welcome message template
MESSAGE = "{} Welcome to [Pyrogram](https://docs.pyrogram.org/)'s group chat {}!"

app = Client("my_account")

# Filter in only new_chat_members updates generated in TARGET chat
@app.on_message(filters.chat(TARGET) & filters.new_chat_members)
async def welcome(client, message):
    # Build the new members list (with mentions) by using their first_name
    new_members = [u.mention for u in message.new_chat_members]
    # Build the welcome message by using an emoji and the list we built above
    text = MESSAGE.format(emoji.SPARKLES, ", ".join(new_members))
    # Send the welcome message, without the web page preview
    await message.reply_text(text, disable_web_page_preview=True)

app.run()  # Automatically start() and idle()