hypeJunction/elgg-followers — comment on commit a05c168b980cb9ddaebecd90995a0347f70cb390
File:  line 0
By hypeJunction:

This plugin makes a distinction between friendship and followership. Similarly, it could differentiate between group membership and followership. A user could follow a group without joining it (provided the group is open).
I suppose the main purpose of 'following' is to get an aggregated river, so populating the 'following' feed with both users and groups being followed would make sense.