You could set up your assistant with OmniFocus and a Mac or an iPad, set up sync, and go at it hammer and tongs. You might not want your assistant to have full access to everything in your database, but selective sharing isn't possible (and a good assistant probably knows all your secrets!) The big sticking point is the possibility of sync collisions. If you and your assistant happen to edit the same action at about the same time, one of the changes will be lost (if you happened to make an identical change, you may not notice the loss). Of course, this is also possible if you use OmniFocus on multiple devices as a solo practitioner, but it is relatively unlikely that you are making changes simultaneously on two different devices, whereas one would expect to have simultaneous changes happening in the multiple-user case.
If you have some convenient means of arranging that you aren't going to be stepping on each other's changes (perhaps she handles putting everything in the database, and you just mark things off as done) it might not work too badly, but you'll want to be careful, as it fundamentally hasn't been built as a groupware application. There are some collaborative features planned that should make this a bit easier, but there's no public timeline to let us know when to expect them.
One safe way that you can do some of this now is with the
send-to-omnifocus@omnigroup.com email service. You send a message to that address, and it sends you back a message a few seconds later with the contents turned into a magic clickable link that will add the contents to OmniFocus as an action via the Quick Entry feature. Your assistant could toss things over the fence to you by sending email to the service, then forwarding the result to you. Another option is to use OmniFocus' ability to process incoming email into actions (see the help for "processing mail messages into actions" in OmniFocus) to allow her to add tasks to your database. It wouldn't allow her to organize projects for you, for example, but she could insert tasks like "Joe Smith called, needs update about XYZ project" into your task lists.