Ila Mitra (Bangla: ইলা মিত্র) (1925-2002) was a legendary peasants movement organizer of the Indian subcontinent, especially in Bengal (now Bangladesh). She was born to an upper middleclass Bengali family in Kolkata on 18 October 1925. She became a communist during her youth. In 1945, she married Ramendra Mitra, who was an active member of the Communist Party despite his lineage from a zamindar (landowner) family of Chapai Nawabganj.
Seelan Palay and his Ila Mitra band members are pro-SDP and had participated in various SDP events, such as the IMF-World Bank distribution of flyers:

That makes Seelan Palay and friends supporters of Communism. Perhaps that's why Seelan Palay has not been given the SDP membership.
Image Source: http://blogs.myspace.com/ilamitra