Unfortunately, the Delegated Admin Extension does not allow these Delegated Admin users to manage user groups.
If you would like to accomplish this behavior, it would be possible to invite them as a Tenant Administrator. And using the Authorization Extension to manipulate user groups/roles/permissions.
Inviting them as a tenant administrator has the apparent drawback of exposing your tenant to them. Therefore, if this is undesired, there will be no way to invite them to manage user groups.
In the meantime, I am collaborating with other team members to see if there are alternative solutions without exposing the entire tenant to the administrator user. If new information arises, I will relay that information to you.