I don't know about any organized afternoon playgroups, but I know the parks around where we live (UC) tend to be pretty busy that time of day, so its a great place to take your lo and meet people. We started walking to the park long before DD could get down and play, but she loved to watch the kids play and she would swing for awhile. And on the first Tuesday, The Rueben H. Fleet Museum is free--there is a great kids area upstairs where lots of moms and kids go--just watch out for the germs. When I took DD there at around 8 months, she got sick two days later.
I know how hard it is with all the morning groups--we go to one on Thurs mornings when we can, but miss it about half the time because of naps (although we are starting the transition to one nap, so in a few months it should be way easier).
~Kim