Allow an "owner" member of a private group to add new members

We have a group of "owners" of a private group who need to be able to add new members. I thought by making these select members "owners" that it would enable them to do so, but all this has done is given them access to admin area. It doesn't give them permission to add members to the group. Does anyone have a solution? I've even tried to add them to a "role" that enables them to do this but it hasn't worked either.