If you are climbing 1000 feet in a 15-mile ride, that's a moderately hilly ride. Considering that you haven't been doing this long and that your weekly mileages have until recently been quite low, 15 mph is a very respectable average speed.
Hill repeats will certainly boost your fitness and increase your power. But if you want to join a group, I'd just join one. The worst thing that can happen is that you can't keep up. However, you will learn some valuable skills by riding in a bunch, and you'll be surprised at how much easier it is to maintain a high speed when riding in a bunch and taking advantage of the draft. But make sure the group you join is aware of your inexperience. Riding safely at close quarters needs a bit of practice and having someone there who is prepared to explain stuff to you will be a big help.
And yes, keep riding more.