My girl went to playgroup since 6 months. Ican feel her development is better, and parents also learn what to do with the baby to enhance her development.
my child starts going to TT and woodland playgroup from 10 months, i think she enjoys a lot especially there are some exercise you will not able to let her try at home such as messing up everywhere playing water, playing sand etc. i think you can try some PG with more sensations exercise, or you can try some baby gym too! but dont expect the baby can learn a lot, aim for making a fun time for the baby is good enough.
I brought my son to playgroup since he was 8-9 month old. At that time, we feel like it's more for me and my husband to learn how we can play with my son. After he could walk, it seems he enjoyed more. Some classes that provide sensations exercise might be good to start with.
for 6m babies, PG like sensory stimulation or gym is ok; but for language.... i think it's too early!! i think it's time for them to learn their 1st language first. i will let my kid to learn the 2nd language after 1y.