it really depends on the type of playgroup you are talking about.
for informal playgroups, where it's more for the mums than the babies, anytime.
for formal "paying" playgroups it again depends on their recommendations. at mine, we accept from 10 months, but most are closer to one year (and i tell parents that when they ask how old their child must be). personally, i don't have a problem for babies that are 10 months coming to class, but i believe that if they are just a little bit older, they will learn and more importantly, enjoy it so much more!