I think to fully grasp what the Lord was speaking to 'the people' through the prophet Isaiah, we have to look back at what He had previously said to understand what He meant by "you are my witnesses".
Is 43:8-9
8 Bring out the people who are blind, yet have eyes,
who are deaf, yet have ears!
9 All the nations gather together,
and the peoples assemble.
Who among them can declare this,
and show us the former things?
Let them bring their witnesses to prove them right,
and let them hear and say, It is true.
After verses 8 and 9 we have the verse that God declares 'His' witnesses. So He is contrasting His witnesses against other 'people' that have 'witnesses'. But to understand why He is making this statement we have to look back even further.
Is 43:1-7
But now thus says the Lord, he who created you, O Jacob,
he who formed you, O Israel:
“ Fear not, for I have redeemed you;
I have called you by name, you are mine.
2 When you pass through the waters, I will be with you;
and through the rivers, they shall not overwhelm you;
when you walk through fire you shall not be burned,
and the flame shall not consume you.
3 For I am the Lord your God,
the Holy One of Israel, your Savior.
I give Egypt as your ransom,
Cush and Seba in exchange for you.
4 Because you are precious in my eyes,
and honored, and I love you,
I give men in return for you,
peoples in exchange for your life.
5 Fear not, for I am with you;
I will bring your offspring from the east,
and from the west I will gather you.
6 I will say to the north, Give up,
and to the south, Do not withhold;
bring my sons from afar
and my daughters from the end of the earth,
7 everyone who is called by my name,
whom I created for my glory,
whom I formed and made.â€
In this 'chapter' we see God declaring His watchful love over those who are 'called by His name'. We see that He will bring His 'offspring' from the east, west, north, and south; bringing His sons from 'afar'. All that are called, all that He created for His glory and who He formed and made. Why is He talking like this in in the beginning? Look at verse 1, "But now thus says the Lord...".
So that leads us back to the previous 'chapter'. Lets look at what is said there.
At first there we see many passages that speak of the Messiah to come. We see numerous passages quoted in the New Testament. Then we see a cry to those who 'have eyes and cannot see, ears that cannot hear'. And in chapter 42 verse 9 we see the 'change over'.
"Behold, the former things have come to pass,
and new things I now declare;
before they spring forth
I tell you of them."
Here we see God declaring 'new' things that are about to come to pass.
"For a long time I have held my peace;
I have kept still and restrained myself;
now I will cry out like a woman in labor;
I will gasp and pant."
Again, He is declaring something that is about to be 'birthed'. Then we see the final verses leading up to the next chapter. In reading/studying passages like these, we must not get tripped up by chapters. Sometimes they do indeed make a distinct separation in thoughts, other times they are just a break in the thought process.
Is 42:18-25
Hear, you deaf,
and look, you blind, that you may see!
19 Who is blind but my servant,
or deaf as my messenger whom I send?
Who is blind as my dedicated one,
or blind as the servant of the Lord?
20 He sees many things, but does not observe them;
his ears are open, but he does not hear.
21 The Lord was pleased, for his righteousness’ sake,
to magnify his law and make it glorious.
22 But this is a people plundered and looted;
they are all of them trapped in holes
and hidden in prisons;
they have become plunder with none to rescue,
spoil with none to say, “Restore!â€
23 Who among you will give ear to this,
will attend and listen for the time to come?
24 Who gave up Jacob to the looter,
and Israel to the plunderers?
Was it not the Lord, against whom we have sinned,
in whose ways they would not walk,
and whose law they would not obey?
25 So he poured on him the heat of his anger
and the might of battle;
it set him on fire all around, but he did not understand;
it burned him up, but he did not take it to heart.
Now, bring this back full circle. Look at verses 8 and 9 in chapter 43. God is speaking of those who talk like they 'know it all'. He is giving them the opportunity to 'plead their case'. But He makes it known that even though they rely on the 'former things', they do not even show by their actions that what they believe in is true. Their 'witness' is a false one, and it is open for all to see.
In verse 10 He then makes the assertion that they were indeed His witnesses. But they are doing a lousy job of it. He makes the plea here in verse 12 about how He came to them in the very beginning.
"I declared and saved and proclaimed,
when there was no strange god among you;
and you are my witnesses,†declares the Lord, “and I am God."
Now that we have come full circle, lets pull this all together. Look at what He declares next.
Is 44:1-5
“But now hear, O Jacob my servant,
Israel whom I have chosen!
2 Thus says the Lord who made you,
who formed you from the womb and will help you:
Fear not, O Jacob my servant,
Jeshurun whom I have chosen.
3 For I will pour water on the thirsty land,
and streams on the dry ground;
I will pour my Spirit upon your offspring,
and my blessing on your descendants.
4 They shall spring up among the grass
like willows by flowing streams.
5 This one will say, ‘I am the Lord’s,’
another will call on the name of Jacob,
and another will write on his hand, ‘The Lord’s,’
and name himself by the name of Israel.â€
So in other words, to make a long story short, He was speaking of the Nation of 'Israel' in 43:10, but He was doing so in forethought of ALL who would call upon the name of the Lord, and eventually be witnesses in Spirit and in Truth.
Now all of those who are the Lord's people are His servants, and they are His witnesses. Not just a chosen few, and definitely not a particular group of people.