Is there any way to sign up for their Gamers Club online or do I have to do it in store?
Most any game that says Mario on the front and is under $15 is a good deal.
I signed up for it online via their website. All you need to do is sign up for an account on bestbuy.com (which is free), and then sign up for My Best Buy (it used to be called Reward Zone, and it's also free if you have a bestbuy.com account). Once you have My BestBuy going, log into it and go to your account options and enable Gamers Club, it's pretty straight-forward.
There are a few different versions of Gamers Club, if memory serves me. One is totally free (which is what I have), and another version i think is called Gamers Club Unlocked, which costs something like $120
per year every 2 years (thanks StryderOmega). There are a number of benefits to having an Unlocked membership, such as 20% off all new games online + in store, among other things. So, it's up to you which one you want. Personally, I stick with the free one just because I don't go into their B&M stores very often (the closest one to me is a ~35 min drive each way), and plus even with the free membership I am still able to get in their online deals 95% of the time, and from time-to-time they do give me some free Rewards Points that you can cash in for discounts, or even sometimes they give free certificates that are good for like $5 off.
Hope this helped you out a little.