The iliopsoas muscle is the strongest hip flexor group and consists of two muscles – the iliacus muscle and the psoas muscle. These muscles are separate in the abdomen but joins in the femur at the lesser trochanter.
The rectus femoris muscle is one of four quadriceps muscles, but is the only one of that muscle group that crosses the hip joint and acts as a hip flexor (as well as knee extensor). Along with the sartorius muscle it’s the second most important hip flexor group.
The tensor fascia latae muscle is a gluteal/thigh muscle located on the outside of the hip that connects to the iliotibial tract (which contributes to lateral knee stabilization).
Adductor muscles in the medial thigh also act as hip flexors, namely the pectineus, adductor longus, adductor brevis and gracilis muscles.
Stretching the hip flexors is pretty much covered by Half-Kneeling Hip Flexor Stretches and its variations.