Some say Bharadvaj Rishi was a main contributor to the Saamveda. But the vedas that we know are composed by Rishi Ved Vyasa. So how Rishi Bharadvaj is related to Saamveda ?
Note: This is the partial answer which explains it's association with Rigveda and possibly with Samaveda.
Visit How did the Vedas in Hinduism come into existence? and some information on Rigveda.
Bharadwaja is the Drashta (seer) of 6th Mandala of Rigveda. There are 765 mantras of Bhardvaja. Bhardvaja's sons are also seers of some vedic hyms. Also note that most of the Samaveda verses are primarily derived from Rigveda.
अविन्दन्ते अतिहितं यदासीद्यज्ञस्य धाम परमं गुहा यत्।
धातुर्द्युतानात्सवितुश्च विष्णोर्भरद्वाजो बृहदा चक्रे अग्नेः॥ १०.१८१.०२
These sages found what lay remote and hidden, the sacrifice's loftiest secret essence. From radiant Dhātar, Savitar, and Viṣṇu, from Agni, Bharadvāja brought the Brhat.
Bharadvaja has assimilated Brihat-Sama & he was Sama-Gayaka
From Aitareya Brahmana:
(He now repeats) a hymn, addressed to Brahmanaspati. * Qandndm tvd ganapatim havdmahe (2, 23) Brahma is Brihaspati ; by means of Brahma he thus cures him (the sacrificial man, who had been torn to pieces). The verses beginning with prathascha L461 yasya saprathascha ndma (10, 181, 1-3) are the three Gharmatanu^ mantras ; by repeating them the Hotar provides the Pravargya man with a body, and a form. (For in the fourth pada of the first of these verses), there is said : " Vasia- tha brought the Rathantara Sama," and (in the last half verse of the second Gharmatanu mantra is said), " Bharadvaja made the Brihat Sama out of Agni." ^ By repeating these mantras, the Hotar provides * the Pravargya man with the Rathantara and Brihat-Samans (required for its prosperity).
Brihat Sama is considered supreme among Saman hymns (BG 10.35).
Gautama, Vamadeva, Bharadvaja and Kashyapa are considered four primary Saama Gayaka (singer). Note: Brief information on Saama Gana.