In Nell's version of the Primer she saves the mouse army and thus they are indebted to her.
I assumed that each of the mouse army had her own Primer which ran its own story arc. This seems to be the case since Nell doesn't interact with Fiona Hackworth and Elizabeth Finkle-McGraw in the Primer world.
What real-world service did Nell do to the mouse army that caused them to seek her our and follow her in the real world?