。
She's singing along to the guitar.
Literal
She [topic-は] guitar [in-time-with-に合わせて] is-singing.
~に合わせて ('in time with, matching, to fit X') is the te-form of the transitive 合わせる ('match, fit together') used adverbially. Common contexts: 音楽に合わせて踊る ('dance to the music'), 時計に合わせる ('synchronize watches'), 相手に合わせる ('go along with someone, accommodate'). Here it specifically means singing in time with the guitar's rhythm and tune — a musical accompaniment. The ~ている marks the action as ongoing, an in-progress performance.