Kasumi is not a playable squadmate in this game. What she means by joining to help you is that she will help with your War Assets with the Crucible project.
If Shepard secured Kasumi's loyalty during the previous game, she will admit that Bau's Alliance black ops raid is the dangerous secret that was hidden within Keiji's greybox. It was carried out on batarians who had come across Reaper technology. That was why Keiji warned that the Alliance could be thrown into war if the secret got out. Furthermore, she will reveal herself after the console's explosion, describing it as a perfect way to fake her death and escape Bau's pursuit, and agree to lend her assistance to the Crucible project.
If Kasumi was not loyal, she will not discuss the contents of the greybox (she doesn't know them), instead complaining that Shepard refused to help her six months ago, and claiming that she still has personal business to conclude. After the explosion, she will also not reveal herself or join the war effort.
my answer showing WHO the playable squadmates are