1. What is the concern about Muslims calling others (e.g.: Anbiya and Auliya) other than Allah for certain things?2. Is there groups in Islam like Sunni, Mujahid, Jamaat etc, or is it one?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Dear brother,

Seeking help from anyone (Prophet, angel, pious person, Wali) other than Allah is major Shirk (polytheism).
Allah Says (what means):

• {And whoever invokes besides Allah another deity for which he has no proof - then his account is only with his Lord. Indeed, the disbelievers will not succeed.} [Quran 23:117]

• {And do not invoke besides Allah that which neither benefits you nor harms you, for if you did, then indeed you would be of the wrongdoers.} [Quran 10:106]

The Prophet said: “If you are asking (for something), ask Allah; if you are seeking help (from anyone), seek it from Allah.” [Ahmad, At-Tirmithi and Al-Bayhaqi]

The Prophet also said: “I shall not be called for help. Indeed, it is Allah The Almighty, The Great, Who should be called for help.” [At-Tabaraani]

2. As for your second question, please refer to Fataawa 81124 and 82706.

We would like to tell you that the person should stick to the Muslim group and avoid anything that could lead to disunity or hatred among Muslims. This meaning exists in many Ahadeeth like the following Hadeeth which was reported by Al-Bukhari and Muslim.

Huthayfah ibn Al-Yaman narrated: “People used to ask the Prophet about good, but I used to ask him about evil lest I would fall in it. So I said, “O Messenger of Allah! We (Arabs) were leading an extremely ignorant and evil life before Allah brought us this ultimate good (the religion of Islam); will there be any evil after this good?” The Prophet said, “Yes!” I said, “Will there be any good after that evil?” The Prophet said, “Yes, but that good will be tainted (not purely good).” I asked, “How would that be?” The Prophet said, “There will be some people who will lead (people) according to principles other than my tradition. You may approve of some of what they say, but you will also disapprove of some of what they say.” I asked, “Will there be any evil after that tainted good?” The Prophet said, “Yes, (there will be) some people who will call to that which leads to Hell; whoever will respond to their call will end up in Hell.” I said, “O Messenger of Allah! Would you describe what those people are like?” The Prophet said, “They will belong to our race and will speak our language.” I said, “What would you advise me if those people came to exist while I am still alive?” The Prophet said, “Stick to the Muslim mainstream and its ruler.” I said, “But suppose there would not be a united group of Muslims or a ruler that could unite them?” The Prophet said, “Then turn away from all sects even if you were to cling to the root of a tree until death comes to you while you are in that state.” [Al-Bukhari and Muslim]

The main theme mentioned in the Hadeeth is the true Muslim group and its leader. This group is the one that sticks to the right path in creed and deed. It is the group that follows the Prophet Muhammad in everything.

For more benefit on the saved sect, please refer to Fataawa 25101, and 22612.

Allah knows best.