Is there to make all of the parts within a group change transparency in sync?

#For Loops

You can use a GENERIC For loop (aka For each loop). It loops through a table using pairs() or ipairs(), here is the format when using the generic for loop:

for index, value in ipairs(t) do -- For each pair of index and value in the table...
	print(value) -- Example (Print the value)
end

Note that you can use _ instead of index if it is not necessary, but it is also good to keep the index variable just in case you need it in the future.

Now, you may wanna put all your parts in a Folder object in Workspace. Then, let's use :GetChildren() to get the list/table of parts you need to change with. Also, make a SERVER Script and put it in the folder you just created

local ListOfParts = script.Parent:GetChildren()

As you can see, I assigned the list to a variable named ListOfParts.

Now, use the generic For loop to loop through the list of parts.

local ListOfParts = script.Parent:GetChildren()

for i, part in ipairs(ListOfParts) do -- ipairs goes through lists like :GetChildren() arrays in order from the index 1. The variable "part" is the value.
	part.Transparency = 0.8
end

Since your wait times are different and you wanted to change the transparency to different values, let's try to make a NUMERIC For loop because the Transparency follows a pattern.

In case you don't know, numeric For loops look like this:

for i = 1, 3, 1 do -- The code is repeated 3 times, from 1 to 3 (incremented by 1).
	print(i, "thing(s)") -- Prints the current number (of times) in "i" and "thing(s)"
end

And in your case:

local ListOfParts = script.Parent:GetChildren()

for i = 0.2, 0.8, 0.2 do
	for _, part in ipairs(ListOfParts) do
		part.Transparency = i -- Continues to be increased by 0.2 until it reaches 0.8, starting from 0.2 itself!
	end
	wait(0.2)
end

We're not done yet! That's just part of the final code!

Now, let's make the whole script:

local ListOfParts = script.Parent:GetChildren()

while true do
	for i = 0.2, 0.8, 0.2 do
		for _, v in ipairs(ListOfParts) do
			if v:IsA("BasePart") then -- Check to see if this is a part, because the Script is counted as a child in the list as well.
				v.Transparency = i
			end
		end
		wait(0.2)
	end

	wait(0.2) -- The other 0.2 seconds are included in the loop. So that's 0.4 - 0.2 = 0.2

	for i = 0.8, 0, -0.2 do -- You can use numeric for loop for decrements as well! In that case, the value of "i" starts from 0.8 and keeps on decreasing until it reaches 0!
		for _, part in ipairs(ListOfParts) do
			for _, v in ipairs(ListOfParts) do
				if v:IsA("BasePart") then
					v.Transparency = i
				end
			end
		end
		wait(0.2)
	end

	wait(0.8) -- The other 0.2 seconds are included in the loop as well. So that's 1.0 - 0.2 = 0.8
end

If you have any questions, please leave them in the comments! Thanks and I hope this will help! If this works, click "Accept Answer" so we can both get some reputation! :)