Dissonance of the Nexus DLC
Sachi won't be added to your friend list until the very end of the DLC main story quest.
First, turn on the new
Increased Affinity Gain setting at the
Main Menu screen, selecting
Options and then LB to get to the far right tab for
Other Settings where you can change your difficulty level (easy/normal/extreme) and now you can have
Increased Affinity Gain (on/off). (This is a new setting from Title Update 1.6.0)
After that, you will need to raise her affinity in order for a green chat bubble to appear over her head in the Lobby in Kirito mode. (*minimum affinity required might be 2, but this needs confirmation as I waited until my affinity with Sachi was 100% into lvl 4)
I would recommend raising her affinity in the DOTN DLC in the White Frontier using the method I describe in my guide for the Yuna, Eiji, and Seven sleepover related achievement:
Once the affinity is high enough:
1-save your game
2-log out to the main menu
3-select Start a New Game (instead of Continue)
4-select your current save slot
5-select Kirito Mode (when the available game mode/DLC mode options pops up)
6-once it loads, fast travel to the Lobby and you will find Sachi in the back on the right when loading in from the Lobby main entrance
(if she doesn't have a green bubble on the mini map then your affinity isn't high enough)7-Once you are allowed to respond, select the top option (the most positive) and boom, sleepover.
The achievement should pop a few seconds after the sleepover scene has started.
As always let me know if anything appears to be wrong, or how the guide can be improved.