I get pain in groin/leg where they meet it feels like needles penetrating it in the dhanur asana only and that stretches pretty much. for me slow stretching works and swan asana is good for open up.
Many asanas can be done in presteps and then fully on this and dhanur i.e. One leg at a time can be done followed by both legs as thill will also warm these muscles up before full pose and will also give some extra protection against injuries. So if some people get pain just tell them to do 1 leg at a time to build up streng slowly.
I think in this pose they tense their muscle at the same time as lifting as it is a difficult pose and then stiffness and tensing at the same time make their muscle cramp as the correct muscle is not lifting and supporting it.
You could make it easier for you and first make one leg lift for whole class and then full pose and tell the new students to skip both leg lift if they feel pain, it is possible for somethat doing one leg will warm them up so full pose does not hurt. All student are different so you better try and see if this works.
The program im doing has This sequence
1 leg lifting both sides.
Shalabasana
Dhanurasana 1 leg both sides
Full dhanurasana.