As the size of your team grows, starting at 15 and perhaps up to 50, you'll see a team structure similar to Figure 2. You may also start to see some of the coordination groups of Figure 3, particularly if you organize your team into a collection of subteams. As the team size grows, the independent tester(s) move from being optional to mandatory due to the complexities of the domain problem being addressed.
If you're working with relational database technology, you'll want an agile database administrator (DBA) who is skilled in techniques such as database refactoring, continuous database integration, database regression testing, and agile data modeling. Small teams will generally spread this work amongst themselves, but as the team grows it makes sense to have someone in the role to focus on data issues. Agile DBAs work closely with developers, ideally pairing with them, to do database-related work. Greater detail about this role and the requisite skills can be found at the Agile Data site (www.agiledata.org).