Frames and components have come a LONG way since the 70's and 80's. That bike was given to you "gratis" because that's all it was really worth. So, I wouldn't drop a lot of money into it other than what you need to maintain it and keep it running.

Keep riding. Do the group rides you suggested. See what you like and don't like about your rig. Then (within a year or so) upgrade to a nice, modern bike. Use your "classic" as a spare or rainy weather bike.

Bob