She was an extra in it.
The character wasn't an actor or aspiring actor like Millman, she was an unskilled background artist.

I imagine it's a mix of the BBC not wanting an untested actor in a big role (Andy came with the pitch), and Maggie being a good friend but not someone you'd want to work with on such an important project.

I'm fairly certain that in Season 1 Maggie states she has no interest in being an actor, and was happy to just be an extra.

I think Maggie is actually self aware. She isn't that keen about being a big star. And its not like Andy is in full control of the show, the producers won't be dumb enough to put her in a major role.

I agree, Maggie could have been the part of KImberley in 'When The Whistle Blows'.
I think for the sake of the other storylines Maggie was involved in throughout, it was crucial that she NOT be in the show, due to the big theme of Andy forgetting where he came from and leaving his friends behind.
Twitter: @kieranthekiz

Once Andy gets money and fame in season 2, he also just wants to mingle with the rich and famous; he never was very modest! He "forgets" who his real friends are, a theme running all through season 2 right until the finale where he finally seems to realize fame can be toxic and he's cut Maggie (who really cares about him) short.

YOU CAN'T HANDLE THE TRUTH!

Because she always beep everything up for him?