Who were the attackers attacked on Amarendra Bahubali when Kattappa killed him?

I want to know who are those attackers?

They are Kalakeya, who Amrendra and Bhallaldeva fought war with in last movie. Their leader was the brother of Kalkeya chief who apparently was killed by Bhallaldeva in last movie.

This role was played by Charandeep.

According to this, this and this,

Charandeeep will be playing a full-length character in Part 2, and his clash with Prabhas will be epic, said unit members. Charan Deep revealed that he would start shooting for second part.

For your other question

Are they men of Bhall?la Deva i.e., was this fight planned by Bhall?la Deva?

Given that Bhallaldeva has fought the war and killed many Kalkeyas, they couldn't be his men nor this fight was planned by him. They appeared to be attacking Maahismati, but Amrendra and Kattappa prevented it. When every Kalkeya was killed, Kattappa killed him too.

For your doubt about Kattappa being obeyer of Bhallaldeva, Bhallaldeva wasn't the king at the time when Bijjaldeva asked him to murder his mother. Kattappa obeyed Shivagami Devi and he was called obeyer(paaltu kutta (pet dog) in Hindi) of Rajmata. When Bhallaldeva became the king, he had to obey him and therefore murdered Amrendra on his order. That's why Bhallaldeva said that Tu mera hi kutta hai (you're obeyer of mine).