As a group (work, family,...), you produce an Aperture library on your computer.
Others (spouse, coworker...) in the group now want to access and edit this library from other computers.
Put the library file in a folder that is accessible to other computers and set permissions so that it can be accessed/edited by other users. You also need to be certain that only one person is editing the file at any given time – the software isn't designed as a shared database and I don't know what, if any, protections it provides against this scenario.
You can also use referenced images instead of managed and put those images in a shared space. However, other users will only have access to the master images and you will want to beware of external edits that are destructive to those master files. Changes to that file hierarchy itself (file moves, renames, etc) made outside of Aperture will result in broken links and you'll have to relocate the images from inside Aperture. . I prefer the simplicity, security and beauty of a managed library.
In what way is this lock-in? It's a simple case of access to a database file. Granted the file — a sparse bundle containing images, metadata and edit instructions in which your master images are stored in their original format — is proprietary to Aperture but the master files remain available should you lose Aperture for some reason.
Accessing the files contained within the Aperture Library package using a right (control)-click
Inside the library bundle your master images are filed, hierarchically by import date, in their original format. They can be copied out as you wish.
I fail to see this as an example of Apple forcing the eco system on you. You are not forced to use Aperture if you don't like it's features and limitations and departure is a simple export (of versions and masters) or "unbundling" of the library file (masters only) away.