Ad populum (or appeal to popularity) fallacy and appeal to authority fallacy are similar in that they both conflate the validity of a belief with its popular acceptance among a specific group. However there is a key difference between the two:
An ad populum fallacy tries to persuade others by claiming that something is true or right because a lot of people think so.
An appeal to authority fallacy tries to persuade by claiming a group of experts believe something is true or right, therefore it must be so.