Every summer, Musica Spei offers a series of informal, just-for-fun early-music “sings” open to anyone who wants to participate. Here’s how it works: Wednesday evenings from mid-July to mid-August, interested singers and listeners join us, in the resonant splendor of St. Anne Church, to sight-read choral works from the medieval and Renaissance periods. Sitting in a circle, with a member of Musica Spei giving pitches and generally keeping time, we sing our way through pieces from our own music collection and from the early-music collection at the Sibley Music Library.