How can we as Muslims unite?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Holding fast to the covenant of Allah and the sunnah of His Messenger (SAW) constitutes the best and nly means by which Muslims can build union, harmony and concord among themselves.
Allah says: {And hold fast, all of you together, to the Rope of Allâh (i.e. this Qur'ân), and be not divided among yourselves,……} [2:103]
Reunion cannot be achieved without faithful return to Shari'a in word and indeed.
Allah says: {And in whatsoever you differ, the decision thereof is with Allâh (He is the ruling Judge). (And say O Muhammadto these polytheists:) ....} [42:10]
{........(And) if you differ in anything amongst yourselves, refer it to Allâh and His Messenger (SAW), if you believe in Allâh and in the Last Day. That is better and more suitable for final determination.} [4:59]
The present disagreement among Muslims contradicts the teachings of Islam, the religion of truth and reunion. Furthermore, Islam considers disagreement as a form of Shirk (polytheism). Allah says: {….and be not of AlMushrikûn (the disbelievers in the Oneness of Allah, polytheists, idolaters, etc.). Of those who split up their religion (i.e. who left the true Islâmic Monotheism), and became sects, [i.e. they invented new things in the religion (Bid'ah), and followed their vain desires],…..} [30: 31, 32]
There are so many Qur'anic verses and Prophetic Hadith that urge Muslim to be united and to avoid dissension. So, Muslims are required to achieve the best state of accord even in small issues if possible. Look! Muslims who perform the five daily prayers in one group, it is a form of unity: One God, One Qibla and one way of life.
Here, we state that disagreement among Muslims is of two kinds:
1- Dispraised difference. This includes dissension concerning belief and all matters that are proved in the Qur'an and Sunnah and consensus of al-Salaf. Whoever falls into this category is no doubt devious and is not of the Muslim fold since he uses his mind in preference to the truth.
2- Acceptable disagreement. This includes all Fiqh and sub-essential issues, since there is lack of agreement among scholars on such matters, and this is what we see in the four schools of Islamic Fiqh. It is a good form of difference, especially if it comes as a result from discretion and good knowledge in Shari'a and the Qur'an and Sunnah.
So, all will be rewarded for their good and faithful independent judgement.
The Prophet (Sallallahu Alaihi wa Sallam) said: "When a ruler (a judge) gives a ruling due to independent judgement he will be rewarded in double if his judgement is found to be right, but will be rewarded once if his judgement is found to be wrong." [ Bukhari and Muslim ].
So, independent judgement among the Prophet (Sallallahu Alaihi wa Sallam)'s companions and their followers resulted from and in respect and honor for one another, rather than accusations of innovation.
Allah knows best.