When do play groups become beneficial for babies?

Not so much . Playgroups are not especially useful for development or socialization for babies less than a year of age. Playgroups are for parents to get together and share their new world of parenting. They are for forging new relationships with other families that are meaningful given the context of your changing family. Playgroups are helpful of course for stay at home moms, but dads are welcome to jump in!
They are... ...Beneficial at all ages, but i think after 6 months, in immunized children, the benefits of social interaction begin to outweigh the risks of infections.