Is there an adjective describing some person or group of people who blindly follow a leader figure, especially with negative connotations?

The obvious, off-the-cuff answer is "lemming" or "lemming mentality," but I suspect that's a little cliché and/or informal for your purpose.

The word "ovine" is defined by oxforddictionaries.com as

Relating to or resembling sheep

There's actually a word sheeplike which has a definition to mean exactly that.

It is in such sheeplike behavior that such an election can arise.

It is in the existence of such sheeplike people that such an election can arise.

Macmillan:

sheeplike

ADJECTIVE SHOWING DISAPPROVAL

behaving like sheep, especially in a way that shows an inability to think for yourself

They demand sheeplike obedience.

A relatively recent coinage is sheeple. It means exactly what you want, albeit a noun rather than an adjective.

The word appears to date back to the 1940s, but I've come across it several times in social media lately, usually in the formulation "Wake up, sheeple!"

It hasn't made it to spellcheck yet, though.

sheeple or sheople People who are meekly submissive or easily swayed (Sheep + people)

Sheeple (a portmanteau of "sheep" and "people") is a derogatory term that highlights the passive herd behavior of people easily controlled by a governing power which likens them to sheep, a herd animal that is easily led about. The term is used to describe those who voluntarily acquiesce to a suggestion without critical analysis or research in large part because the majority of others possess a similar mindset.

_{-- Wikipedia}

“acolyte” in the sense of follower, has been used for this purpose