Right now, only members can be placed within a group. Some communities use groups to organize members by way of geographical divisions, like sections within their neighborhood. This type of organization is based on the address, which is associated at the account-level.
With the current design, if one member were placed in a group, and they added a second member to their account, that second member wouldn't be part of any group.
If, instead, accounts themselves could be added to groups, new members added to accounts already in groups would inherently be part of those groups too.