The TOS isn't really going to work that way. What you have is essentially two groups of users, the second group being a subset of the first with a second level of requirements (terms). There is not a built in process to handle that.
The framework could handle it, but you'd have to do some customization of your flow. In general, you would need a secondary process for users that wanted to join the second group. You could ask them to update their info and include a terms checkoff at that point, or you just have a terms checkoff process that then adds them to the second group, which I would do with an additional user meta. Then you could validate that user meta in a wpmem_securify filter to determine which group(s) the user belongs to.
There are a lot of hooks in the framework. You can get a list here: http://rocketgeek.com/plugins/wp-members/users-guide/filter-hooks/