Blender does not provide a way for multiple uses to edit the same blend file at exactly the same time. *
However typically you can manage this by splitting your assets into separate blend files and linking from one file to another.
Blender supports recursive linking so a scene, node-groups, animation, models, materials etc can all have their own files.
Here is the documentation for the linking structure use by the open movie; Tears of Steel
Some projects do this more fine grained, where the
material, armature, mesh, action are all stored in 4 separate blend files and they are combined in a 5th, and composited in a 6th. But inter-linking complexity needs to be managed or you have a mess too, where you load a blend file and 100's of megabytes are loaded because files are interlinking in situations that might be avoided.
I have found the best way to manage linking is to only go in one direction.
So for example:
Materials/Nodes -> Models/Rigs/Environments -> Animation -> Compositing
But never link an animation into a material file for eg.
This way its predictable and you can manage what loads in a more controlled way.
Update: since 2.78 its now possible to reload libraries without re-opening the file, see: Datablock and Library Management.
*Blender used to have Verse integration, but it was removed due to not being very practical to use for real projects.