The primary hip flexors are a group of 6 muscles; the rectus femoris, TFL, iliacus, psoas, iliocapsularis, and sartorius muscles.
Rectus femoris - also one of the quad muscles
Tensor fasciae latae (TFL) - with the gluteus the TFL does wide variety of hip movements, including flexion, abduction, and internal rotation.
Iliacus - flexes and externally rotates the femur
Psoas - joins upper body to lower body
Iliocapsularis - stabilizes the hip
Sartorius - both a hip flexor and knee flexor