When your organization is in the Plus Tier, you have all the options available in Standard, with no participation limit. You can also request reports for any Challenges you run with your organization.
Plus Pricing is simple. $2.29/user/Challenge. You are only charged when you run Challenges, and only for those that participate.
If you're ready to upgrade your account from Standard to Plus, simply send an email to email@example.com with the name of your organization.