Salaam
The quranic verse that you have cited was revealed about the Jews and Christians. Al-`Awfi said that Ibn `Abbas commented,
[إِنَّ الَّذِينَ فَرَّقُواْ دِينَهُمْ وَكَانُواْ شِيَعًا]
(Verily, those who divide their religion and break up into sects...) "Before Muhammad was sent, the Jews and Christians disputed and divided into sects. When Muhammad was sent, Allah revealed to him,
[إِنَّ الَّذِينَ فَرَّقُواْ دِينَهُمْ وَكَانُواْ شِيَعًا لَّسْتَ مِنْهُمْ فِى شَىْءٍ] (The verse you cited)
(Verily, those who divide their religion and break up into sects, you have no concern with them in the least.) It is apparent that this Ayah refers to all those who defy the religion of Allah, or revert from it. Allah sent His Messenger with guidance and the religion of truth so that He makes it victorious and dominant above all religions. His Law is one and does not contain any contradiction or incongruity.
Clearly, from the above there is an order by Allah not to divide the religion. In terms of uniting muslims, we can only be united once we endeavour to emulate the ways of the Prophet and their companions. We cannot unite with deviant sects on issues of aqeedah as those who do not follow the ways of the Prophet are destined for a holiday in hell. This message is reiterated by Allah's beloved Prophet Muhammad (pbuh):
Abdullah bin Amar (RA) relates that the Holy Prophet (peace be upon him) said "Surely things will happen to my people as happened earlier to Israelites, they will resemble each other like one shoe in a pair resembles the other to the extent that if anyone among the Israelites has openly committed adultery to his mother there will be some who will do this in my Ummah as well, verily the Israelites were divided into 72 sections but my people will be divided into 73 sections, all of them will be in the fire except one." The companions asked, 'Who are they O Messenger of Allah,' Holy Prophet (peace be upon him) said, "They are those who will be like me and my companions."
The above hadith identifes that even in the religion of Islam there will be sects; a resemblance Muslims will share with the Israelites. Therefore uniting with deviant sects on matters of aqeedah could prove detrimental to oneself.
May Allah keep us on the straight path.
Maximus