This is an interesting can of worms and a lesson in being proactive about securing your trademarks and domain names in other jurisdictions whether you intend to launch there or not.
Despite the fact that Scoopon is arguably a real business this really is a domain squatting case. After all Groupon has nothing to do with their business or branding. Registering the trademark and the domain name was preemptive and is leverage to get bought out. I don't really care if they are a real business or not.
Speaking as an Australian, it is important that companies that operate in Australia have a .com.au domain. It's not uncommon for me to tell someone my address (gmail.com) and then find out later they'd tried to send it to gmail.com.au.
I always thought it was a mistake by the UK to use .co.uk instead of .com.uk for consistency reasons but having thought about it, I think it may make the distinction clearer.
So what should Groupon do? Basically they should do what Google did in the UK. For years Google didn't own the gmail name there so branded themselves as googlemail.
By pressing on you're telling the other side that you're not reliant on them giving up the name. It gives you bargaining power.
Suing them is a good idea too. It bleeds money they probably don't have (which clearly Groupon does).
The worst thing Groupon could do is stall their activities waiting for a resolution. Groupon is a time-sensitive business. There's nothing magical about what they do and someone else can replicate it (as happened in Germany I believe).
It would be risky to proceed under the brand name of Groupon however because the trademark muddies the waters. There is the possibility that if Scoopon successfully defend the trademark, Groupon will then be liable for damages for infringing upon it.
Best bet: pick a new brand name, register the trademark and domain name and proceed. Make the other side an offer with a time limit. If they decline, stop talking to them. While this is happening simply proceed. There is infrastructure that needs to be built regardless of the brand name (finding offices, hiring salespeople and copy editors and so forth).
As you gain traction occasionally make them an offer for the name. As time goes on LOWER the offer because by this point you've invested in your new brand so the old one has less value.
At some point the squatters will either give up and sell or it simply won't matter because you'll have built your business on a new brand.