Q:

Is a Muslim allowed to think some other group of Muslims having different views are unbeliever (Kafir)? 

There are many different views in Islam, but all Muslims have major things in common:

Belief in Allah as the one and only God who created the world
Belief in Quran as the messages of Allah to the human
Belief in prophet Muhammad (PBUH) as the last prophet sent by Allah
Belief in Ka'ba as the unique Qibla to pray towards
Belief in all the previous prophets including Adam, Noah, Ibrahim, Musa and Isa
Belief in the future life, the day of judgement, heaven and hell
And many other things

So this is my question: If there is another group of people who believe in Allah as the unique God, Prophet Muhammad (PBUH) as the last prophet, and the holy Quran, is another Muslim allowed to call them unbeliever (Kafir)? -  By Kafir I mean Kufr-Ul-Kibr as stated in @DanAndrew answer below. 

A:

Common people may not issue the verdict of Kufr on a Muslim, unless he explicitly confirms it. 
A Muslim may be described as "doing an action of Kufr" if that is clearly the case, but that does not make him a Kafir.  An example of such an action would be to abandon praying completely. 
To claim Kufr of a Muslim, an Islamic judge has to go through a number of very strict steps to confirm it.  For example, in the aforementioned case of abandoning prayers, the judge has to make sure the accused understands all the following:

The accusation and its cause. 
The rules that make performing this action obligatory. 
The proofs (Quran, Hadith, Tafseer, Fiqh, etc.)  that make abandoning this action an action of Kufr. 
The accused has to confess that he denies the obligation of such an action, and that he is not leaving it out of laziness for example. 

The rules above are neither comprehensive nor decisive, but you get the idea.