Do you mean adding a project to a folder? (you can create a folder using the + button from the project list) Edit/Outling/Add Child should be disabled when you have a project selected because you can't nest a project inside another project.
Or are you referring to action groups in a project? Action groups can not currently be created on the iPhone, only on the Mac which can then be synced over. Please send an email to
omnifocus-iphone@omnigroup.com if you would like to official cast your vote for this feature. Thanks!