Hello, the lifts usually open mid june, you can ride ride some lower trails in the valley, but it is quite a steep valley, and the climbs probably will kill your mates, unless they are pretty fit, there is also Pila, about an hour away through the tunnel, not sure if that will be open that early, but i would check if i were you. If you can get to st Gervais for the first or last tram up to the col de vosa or just passed it, you will get a good decent down to les Houches, bikes are only allowed on 1st or last tram (funicular).Also the Vert hotel used to do an uplift a couple of evenings a week, they might be worth contacting, as would zero g the bike shop. Good luck, hope you have a great time.