Sangha is a Buddhist word for spiritual community — one of the “three jewels”, along with Buddha (Awakened Being) and Dharma (The Teaching). Traditionally, this “third jewel” was the monastic community. More colloquially, that meaning has spread out to include the broader community of Buddhist practitioners and devotees.
Nowadays, sangha often appears in interfaith vocabulary. In the most general sense, sangha can mean any group of spiritual friends or contemplatives who meditate together, study spiritual teachings, and support each other on the path. In this sense, practicing together is what makes a group a sangha, no matter how diverse the backgrounds or beliefs of its membership.
There are individual “sanghas” — specific groups, with particular histories, identities, doctrines and norms. You might have to formally join (or at least show up someplace) to become a member of one.
There is also just Sangha — the one Universal Sangha of all who walk the contemplative path — past, present and future. You automatically belong to this Sangha the moment you aspire to the path, or find yourself on it.