Lo titgodedu / Náánuim

This commandment also includes [a prohibition] against there being two courts which follow different customs in a single city, since this can cause great strife. [Because of the similarity in the Hebrew roots,] the prohibition against gashing ourselves [can be interpreted] to mean: "Do not separate into various different groupings."

“The order (of prayer) on the Night of Yom Kippur” - Containing six paragraphs.
On the Night of Yom Kippur the custom is that the reader says, “In the court of high, in the court of low (earthly); with the consent of God and with the consent of the congregation, we are permitted to pray together with the transgressors”, and it is customary that he say “Kol Nidrei”, (“All the Vows”) (and the entire prayer that follows) and afterward he says (the prayer) “שהחינו” without a cup (of wine).
Hagah: And afterwards they say the Evening Prayer. It is customary to recite “Kol Nidrei” while it was still day and to lengthen it with melodies until nightfall, and (“Kol Nidrei”) is said three times, and each time (the cantor) raises his voice “higher” (says it louder) than before, (מהרי״ל). And likewise the reader says the following prayer sentence three times; “And the entire congregation will be forgiven, (etc.)” And the congregation says three times, “And God said I forgave you according to your word”, (מנהגים). A man must not deviate from the custom of his city even in the melodies or piyyutim that are said there, (מהרי״ל). On the Night of Yom Kippur and on the next day we say (the blessing), “Blessed be His name whose glorious kingdom is forever and ever” in a loud voice (where normally it is said in a wisper). If (Yom Kippur) falls on the Sabbath we say “ויכולו” after the Silent Prayer, and the prayer “תפילת מעין שבע” and conclude (the prayer with the words), “מקדש השבת”, (“who makes the Sabbath holy”), but do not mention Yom Kippur. (We do not say “אבינו מלכנו” on the Sabbath, but the rest of the prayers for forgiveness and the supplication prayers one says like on a weekday, (ריב״ש סימן תקיב ומנהגים),). It is necessary to make one man stand to the right side of the reader and one to his left side (while he is saying these prayers). There are people who stand on their feet the entire day and the whole night. It is customary to spend the night in the synagogue and to recite poems and songs of praise all night.
Hagah: It is better to sleep far from the ark (מרדכי), and whoever does not want to recite songs of praise and poems should not sleep there (in the synagogue), (מהרי״ו). The chanters (cantors) who pray the entire day should not stay awake all night because they might ruin their voices if they do not sleep, (מהרי״ל).

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Some have the custom to cut their hair on Rosh Chodesh Iyyar, but this is a mistake. Rema: However, in many places, the custom is to cut their hair until Rosh Chodesh Iyyar, and they do not cut their hair from Lag BaOmer forward, even though it is permitted to cut one's hair on Lag BaOmer itself. But those places who have the custom to cut their hair from Lag BaOmer onwards, do not cut at all after Pesach until Lag BaOmer. And within one city, it should not be that some follow one custom, and some follow another, because of "lo titgodedu" ("do not cut yourselves" - Deut. 14:1, interpreted in Yevamot 13b as "lo te'asu agudot agudot," "Do not become seperate groups"); all the more so, one may not follow both leniencies.

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