I was completely captivated by that group's performance.
Literal
I [topic-は] that group [of-の] performance [at-に] was-enchanted [emphasis-よ].
うっとり is a 擬態語 (state mimetic) for being entranced or rapt with admiration — typically in a pleasurable, dreamy way. Combined with する it becomes a verb 'to be enchanted.' に marks what enchanted the speaker. The final よ adds conversational warmth.