The word congregant can be used, although it's not Islam-specific.
Actually this question came up on English.SE: “Parishioner” vs. “congregant”, who also offer other possibilities:
To refer more generally to those attending services at a particular time, you could simply say worshippers or attendees; for all adherents, there are a variety of terms employed, such as the brethren or the faithful, or the more mundane churchgoers or the observant; communicants captures the sense of those in communion with the Church as opposed to outsiders.