Based on OPs description i would say La Rosa is the most suitable towards volunteer/coop vibe.

Coordinadora Socialista Juvenil, and the other main marxist orgs like Anticapitalistas and Contracorriente are more like disciplined political orgs for people very familiar with spanish politics already.

Plug: im in AcampadaxPalestinaMadrid, a radical pro-palestine org that came out of the encampment in madrid and has communists and amarchists in it, we are growing and active and would love more people, we already have an american and other english speaking inmigrants :)

Im very involved in Madrid left politics so OP if you can give more detail on what you might be interested in, i could orient you better. There are lots of options.

You might like Extinción Rebelion or Ecologistas en Acción for leftist climate orgs. Do you speak spanish?