As a girl, Annika Hansen was assimilated into the Collective, along with her parents, and named Seven of Nine. Who were the other six people who were assimilated along with them?
Seven of Nine was not assimilated with nine other people. She was assimilated along with her parents, Magnus and Erin Hansen.
At some point in the future, she was then assigned to a working group servicing Unimatrix 01. This group contained nine drones, of which she was the seventh, hence her full Borg name
"Seven of Nine, Tertiary Adjunct of Unimatrix 01"
Your question rests on a faulty premise. It is incorrect to assume that a designation refers to the number of individuals assimilated at the time. Rather, Memory Alpha explains that:
A drone's designation typically described its position within a group, e.g. "Third of Five."
So, there need not have been nine people present at the time of Seven's assimilation! 'Seven of Nine' merely refers to her position in a specific group.
Additionally, as pointed out by @O. R. Mapper in the comments below, in 'The Omega Directive' (VOY), Harry is reassigned by Seven to the designation of 'Two of Ten'. The fact that she reassigns him provides strong evidence to indicate that the designation refers to a position within a group (which can change) rather than a 'name' which it would not make sense to change!
Borg drone designations in the form n of m were established in the Next Generation episode Season 5 Episode 23 "I Borg".
In that episode, the crew found a crashed Borg scout ship. On board of that ship were 5 borg drones of which only one was alive. The surviving drone identified itself as "three of five". The crew inferred that it meant that the drone was the 3rd member of a group of five drones.
Later Star Trek works kept using this naming system for Borg drones. "Seven of Nine" means that she was the 7th member of a group of nine drones.
An alternative explanation could be that the second number is the number of the unit the drone belongs to (7th drone of the 9th group of drones), but there is no canon example of a Borg drone where the first number is larger than the 2nd, so there is no reason to believe that the hypothesis from "I Borg" was incorrect.
There are, however, three examples of Borg which are not named with the n of m naming system:
- "Locutus of Borg", the name assigned to Captain Picard after being assimilated
- The "Borg Queen" (although she might not technically count as a drone)
- "One", the 29th century technology Borg drone from the Voyager episode Season 5 Episode 2 "Drone" (But this designation was given to it by itself after inspiration from Neelix, not by the Borg collective to which it never had any contact)
So there is precedent for the Borg collective giving special names to drones which have a very unique purpose.