Example of how to use the ruby gem xcodeproj to add a library to the Frameworks group of a project. Forked and updated to work with updated version of xcodeproj.

add_lib_with_xcodeproj.rb

require 'xcodeproj'

project_path = "your_project_path";
# Create project object
project = Xcodeproj::Project.new(project_path);

lib_path = "your_lib_path";

# Add the lib file as a reference
libRef = project['Frameworks'].new_file(lib_path);

# Get the build phase
framework_buildphase = project.objects.select{|x| x.class == Xcodeproj::Project::Object::PBXFrameworksBuildPhase}[0];

# Add it to the build phase
framework_buildphase.add_file_reference(libRef);

# Save the project
project.save();