Hi All, We're expanding our development group, and I was hoping t

Every thing depends on architecture of the application.
It should so loosely coupled that different persons can work upon different components. This link (forum.appian.com/.../BPM_Application_Delivery_-_Actualize) will give you an idea to how to decide components so that multiple users can work upon same projects.
#1. So, before actual development, find out the common functionalities in different parts of the process and which users going to utilize and divide the process in number of components.
As you decides you sub processes and main process. The sub process are modeled by different users and the developer of main process directs the sub process node to a dummy PM (having only start and end event with no pv!s). And when the dev of sub process completes only the mapping of variables remains.
To avoid confusion of variables, a common pattern is decided before hand.
Many other activities can be done in parallel like process, dashboard/Records, Reports.

#2. person X assumes that person Y will be providing that data at some point of time when all the mapping as stated in #1 is done.

#3. While in iterative development, the required enhancement should not disturb the functionality of currently running instances. Eg: user should not change the number of parameters of existing rules (which are already in production). This may result that PM is of older version which takes let say 3 input for rule and the rule is modified and take only 2 parameters, then the current running instances will give error.

Hope this helps.