CONGREGATION. The Assembly of people in a mosque is called Jam’ah جمة, the term used in Afghanistan for the mosque itself.
There are special rewards for those Muslims who assemble together for the stated prayers; for Muhammad has said, “The prayers which are said in a congregation increase the rewards of the worshipper twenty-seven degrees.” “Say your prayers in a congregation, for a wolf does not eat the sheep except one has strayed from the flock,” (Mishkat, book iv. ch. ixiv.)
The Sunni style themselves Ahlu Sunnah wa Jam’ah, i.e. “the people of the traditions and of the congregation,” in contradistinction to the Sh’iahs, who do not worship in a congregation unless the Imam, or leader, he a man entirely free from sin [IMAM.]
The word jam’ah is also used for an assembly of people collected to decide a question of law or theology, the ijma’ being their decision, more frequently called ijma’u ‘l-ummah.
Based on Hughes, Dictionary of Islam