Reach the Goblin Camp in Act 1
Speak to Minthara in the Shattered Sanctum
Agree to help her attack the Emerald Grove
Do not kill her during the grove assault
After the raid, long rest until the celebration scene
Continue through Act 1 without killing Minthara
In Act 2, reach Moonrise Towers
Find Minthara imprisoned in the Moonrise dungeons
Free her during the assault on Moonrise Towers
Choose the dialogue option to recruit her
Keep her alive through the encounter
Continue the story with Minthara as a companion