Sand was a passionate admirer of Lamennais’ brand of Saint-Simonist Christian Socialism; she took Liszt to see him after Liszt was put on trial and sentenced for his political views. Châteaubriand and Lamennais were erstwhile collaborators.
Fichte was influential on his thought. Schleiermacher met both Schlegel brothers in Berlin — Friedrich Schlegel had an adjoining apartment, and together they translated Plato’s ‘Dialogues’ (the work is still admired today, though its joint nature led to strains in the friendship, and Schleiermacher increasingly took it over himself). In Jena, he joined the Schlegels and Novalis in founding the influential journal Athenaeum (Tieck was another friend in romanticist circles). He was a colleague of Humboldt’s in founding Berlin University, where his relationship with Hegel was marked by mutual wariness and antipathy.
Many of Niethammer’s friends and acquaintances were part of an extended gang with Jena connections. Schiller was a university colleague there, Hölderlin, Fichte and Novalis are famously supposed to have spent an evening together at Niethammer’s, and on Saturdays, Schiller and Schelling played cards with him. Niethammer was one of Hegel’s closest friends, and helped him get various jobs. Hegel wrote to him admiringly of Napoleon whom he’d just seen — Napoleon’s army was fighting the Prussians just outside Jena, and duly won; helpfully for Hegel, who was trying to finish a book, without closing down the university.
Lavater’s lifelong friend Fuseli and he were briefly exiled for denouncing a corrupt magistrate. Sulzer accompanied them on a journey on which they met Klopstock, and the great Jewish scholar Mendelssohn, whom Lavater unwisely tried to convert to Christianity. Ideas he had developed about the soul led to his meeting Kant and Herder. Goethe became a great friend (though they later fell out), mutually fascinated by the soul’s expression through the face. Among his many correspondents were Rousseau, Pestalozzi, Wieland, Basedow and Claudius. Though he greatly influenced Blake, it is unlikely that they met.
Moore met da Ponte in New York, where his father arranged for da Ponte to take up the position of first professor of Italian at Columbia College. By the way, there is significant dispute about whether or not it was Moore who authored ‘A Visit from St. Nicholas.’