Translating/summarizing from the page you linked to:
- You order the ticket online, everybody must go to the same destination, which you have to enter in the order form.
- You (the “group leader”) receive an email with a “coupon” code and explanations about how each group member can get their ticket.
- Each group member fills in a departure station (you can even come from different places and meet at the destination) and gets an e-ticket to print at home.
In Dutch “retour” means return so you can certainly go back to your point of origin with the same ticket but it's not a day ticket (i.e. you cannot travel freely on the whole network, unlike many German group tickets).
The main conditions are:
- Travelling after 09:00 on weekdays or in the week-end (you can't use it during the morning rush hour)
- Ordering online and printing the e-ticket (it can't be bought or redeemed at the station). This also means you need a Dutch debit card with iDeal, you can't pay by credit card.
Until now, it's a temporary offer, running until August 2014 for travels before October 2014 (I write “until now” because it's been extended once and I imagine it could become a permanent things or maybe a recurring offer).
Note that Dutch railway cards offer a somewhat unusual discount (at least compared to other countries I am familiar with): Under some conditions yearly card holders can take up to 3 other travellers with them with a 40% discount (it's called Samenreiskorting). So if you are travelling in a group with a Dutch resident, you might want to ask them about it, it can also be an easy way to save money.