Normally, you can only have 2 squadmates in play, including dog. Attempting to call one in when you have 2 will generate a message saying that you have too many. Attempting to call one in from a Loki/Odin will result in the squadmate simply not appearing/loading.
Basic cheat: suit up in any suit (maniac/juggernaut/juggernaut recon/probably Michael Myers/Predator too), then use a satellite (Loki/Odin) to call in a squadmate (with 2 already in play), then it will load as long as you did it while suited up.
Assault is unlikely since you would need (with hardline) 14 consecutive kills without dying, 3 times over to get one extra squadmate.
Best bet: 2 classes, both hardline (not necessary, but it helps).
1. Has dog killstreak,
2. Has support squadmate, juggernaut recon, Odin.
Get dog, switch classes (dog will be in fourth killstreak slot). Earn the support streaks.
Call in dog, support squadmate, juggernaut recon care package.
Suit up as juggernaut recon, call in Odin, call in Odin support squadmate.
Now you will have 1 dog, 1 wimpy shield guy (support squadmate), 1 tough shield guy (Odin squadmate), and you in your recon armor for your own little quad squad.
Hardline: 19 kills.
No hardline: 21 kills.
Only the 4-5 kills for the dog need to be in one life. The support ones can be gained over several lives.
Don't cheat online.
[more cheats for Call of Duty: Ghosts here]