In this PDF file, we have an invitation from Hizb ut-Tahrir to the Muslims, it is titled, "A Call From Hizb ul-Tahrir To the Muslims in General and Specifically to the Muslims of Iraq and Lebanon" (dated 10/05/2003) and in which they write:

Does it not cause tears, that the shia do not enter the sunni mosques, and if they entered, the eyes turn at him. And that the Sunni do not enter the mosques of the Shia, and if they entered, it is as if it is a foreign land?

And also:

Indeed, Hizb ul-Tahrir, the political party established upon Islam, and Islam alone, it's membership is open for every Muslim, whether he is Sunni or Shia.

And also:

Just as Hizb ul-Tahrir calls upon the Islamic movements and Islamic parties, not to close their membership for sunnah or Shia.

Notes

Marxists, Communists and Socialists - in order to effect their revolutions and establish the "social justice" as they claim - require the support of the masses. They need a critical mass of people so that their envisaged revolution can take place. This methodology is one that has been taken by the thinkers of the early to mid-20th century. Drinking and gulping this ideology has effects and consequences. It makes a person to inherit hatred towards the Companions of Allaah's Messenger (sallallaahu alayhi wasallam) and to evaluate them and their actions through the scales of the materialist notions of "social and political justice" and to find fault with them on this basis. For this reason you will find hatred towards Mu'awiyah bin Abi Sufyaan and love and closeness towards the Raafidah Shiah amongst factions of these Kharijites of the 20th century, from whom are Hizb ut-Tahrir. This is because they want to amass everyone under their umbrella irrespective of creed and orientation to enable their objectives to be fulfilled with ease.

The founder of Hizb ut-Tahrir was a former Ba'thist Communist. He took his Ba'thist Communist secret party politics and set up an Islamic party on its basis, working upon that same Marxist, Communist methodology he had been up prior. This is the very same model that Hizb ut-Tahrir are operating upon today. It is a Communist, Marxist revolution disguised as Islam. It is very similar to the approach of Sayyid Qutb in his Milestones, who incorporated the manhaj of Vladimir Lenin and concealed it with Islamic terminology.

Wherever you have Kharijites, you will find the Rafidah close by, because the origins of these two sects, the first two sects in Islamic history in fact, have common roots. You will find cooperation between them. Look at Hasan al-Banna, Sayyid Qutb, Mawdudi, look at Salman al-Awdah, look at Ikhwan al-Muslimeen, look at Iyaa al-Turaath and other Ikhwani organizations - you will see cooperation between them and the Shi'ah to one degree or another.