。
She furrowed her brow.
Literal
She [topic-は] forehead [target-に] wrinkles [object-を] drew-near.
額に皺を寄せる literally 'draw wrinkles to one's forehead' is the standard idiomatic phrase for furrowing one's brow. 寄せる ('to bring near / draw close,' transitive partner of 寄る 'to approach') paints the wrinkles as something *gathered* into the forehead, not just appearing there. に marks the location where the gathering happens. This is one of a handful of stock body-language phrases (顔をしかめる 'grimace,' 眉をひそめる 'knit one's brows') that recur in fiction to depict displeasure, concentration, or worry without naming the emotion directly. Skilled writers lean on these because the gesture lets the reader infer the inner state.