Salam.
As per my understanding and according to Quran anyone is monotheist who submits to the One Allah and do good to others is a Muslim. Here it must be clear that the name 'Allah' is only an Arabic name for God. It is not an exclusive name for God.
(please see the related article by Brother Joseph :
http://www.quransmessage.com/articles/allah%20is%20not%20an%20exclusive%20name%20for%20god%20FM3.htm)
As Brother Joseph also wrote in the article I recomended earlier:
"Muslims are those that submit their will to God and can be from the followers of the previous messengers and scriptures (Jews and Christians included)"
And in another article titled WHY IS THE TERM 'MUSLIM' HIJACKED Brother Joseph also wrote:
"From a Quran's perspective, even Jews and Christians can be 'Muslim's if they have solely surrendered their purpose to God. The term believers (Mu'min) is used predominantly for the followers and believers in the final scripture (Quran). Some of these can be from the People of the Book as well. Please see article (2) below and in particular section (17) entitled "THERE ARE 'BELIEVERS' AMONG THE PEOPLE OF THE BOOK".
In conclusion, a Christian or a Jew may be a 'Muslim' but not necessarily a believer or in the case where they believe in the veracity of the Quran, they may be a Muslim and a believer, but choose to follow their own laws (Shariah) revealed to them in their respective scriptures."
(link:
http://www.quransmessage.com/articles/term%20muslim%20hijacked%20FM3.htm)
Now,
So far as I know, Brahmo Hindu or Unitarian Christians and many others like them are monotheist, means, they submit to One God and does not set any partner with Him.
Unitarianism is historically a Christian theological movement named for the affirmation that God is one entity, in direct contrast to Trinitarianism, which defines God as three persons in one being.
(
https://en.wikipedia.org/wiki/Unitarianism)
Brahmo is a monotheistic religion also, It denounces polytheism, image worship, and the caste system. They don't set any partners with God.
(
https://en.wikipedia.org/wiki/Brahmoism)
Please see also the the follwing article :
http://quransmessage.com/articles/people%20of%20the%20book%20FM3.htmHope, I made my point clear.
Thanks.