。
She sat down beside me.
Literal
[topic-は] I [with-と] line-up-and sat.
並ぶ ('to line up, be aligned') used with the comitative particle と describes two or more people positioned side-by-side. The te-form 並んで links 並ぶ to the main verb 座った, giving 'lined up alongside, then sat' — a manner-of-action chain. English flattens this to 'sat beside me,' but Japanese encodes the spatial alignment as a separate verb. The と here is the comitative ('with, together with'), distinct from the quotative と ('that, saying') or conditional と ('when').