The closing of Johnny D’s Uptown in Somerville left a big hole that needed filled: a place for blues musicians to jam on Sundays and support one another. That is the spirit The Porch fosters with its People’s Jam. Bass amp and two guitar amps provided, along with DIs, vocal mics and horn mics. Just come in and sign up. Each group of jammers plays three songs and no one repeats until all players have had a turn. Admission is free. Note: This will be the final People’s Jam at the Porch, which is closing after five years in Medford.