I get that part, but how does that work when other people also contribute changes for which they own the copyright and which are also AGPL licensed, but then to the Aerospike devs?
Well, since the project was just released as opensource, presumably all work on it so far has been by employees of Aerospike (who automatically grant copyright to their employer by virtue of the work-made-for-hire exception).
I see this as one of the fundamental problems with the AGPL/Apache licensing patterns. The company can create an Apache driver, but it's not clear when a member of the community creates a driver if they can also use the Apache license. If all they do is present the driver, then presumably yes, but if they publish a service using their driver calling the AGPL licensed code, their driver should be subject to AGPL's terms which would apply the AGPL to it.
Alternatively they could assign the copyright of their code to a company? But why would they be motivated to do so?
This creates an asymmetry of rights where the company can do things that community members cannot. This seems to the very opposite of what the Open Source movement has been all about.
Most projects like this have a contributor release which either requires outside contributions to either be entirely licensed to the project or have a specific grant allowing the founding company to use it as part of their non-GPLed release.