Ex-Borg Riley says she was assimilated at Wolf 359 (re: Star Trek: The Next Generation: The Best of Both Worlds: Part 2). The Borg ship that battled Federation forces at Wolf 359 later went to Earth and was ultimately destroyed by actions of Enterprise-D, which would suggest that none of the newly-assimilated Borg could have been transported back to the Delta Quadrant. However, 5th season's Star Trek: Voyager: Infinite Regress provides another instance where a character (one of Seven of Nine's personalities) claims to have been assimilated at Wolf 359. This may indicate that during that battle, there were, in fact, two Borg vessels (or more), not just the one that exploded. Another possible explanation is that some of the Federation ships were boarded by Borg who assimilated their crews and then, after their cube was destroyed, headed back to the Delta Quadrant in those ships, using Borg technology to increase their speeds.
When Chakotay links with the group he has visions of memories including some from Riley. One of the images is of Riley playing with her grandfather from a third person's point of view. Rileys memories should be from her point of view.