Sure, right now the number of participants is low but, like all Blizzard betas, more players will be added gradually. If you want in on the Heart of the Swarm beta, you'll need to first opt in to the beta through your Battle.net account, and then exercise patience – this is perhaps the most crucial step. "Signing up from a Battle.net account that's actively playing StarCraft 2 will help your chances of being selected," Blizzard says.
No exact beta period has been determined and, if you do get in, you'll only be able to sample the multiplayer side of Heart of the Swarm, compatible with both PC and Mac. Blizzard will open up the beta to more people through giveaways and fansite promotions eventually, so we'll let you know if more (and faster) opportunities become available.