my_resource_spec.rb

require_relative "./my_resource_spec/shared.rb"

RSpec.describe MyResource do
  describe "#a_method" do
    include_context "with shared foo and bar", lambda {
       it "is great" do
         expect(my_resource.a_method).to eq('is great')
       end
    }
  end  
end

###
### Result
# MyResource
#   #a_method
#     with shared foo and bar
#       is great

shared.rb

RSpec.shared_context "with shared foo and bar" do |block|
  subject(:my_resource) { create(:my_resource) }

  let(:foo) { ... }
  let(:bar) { ... }

  before { ... }

  context "with shared foo and bar", &block
end