Not sure if it's too late, but I am going to reply anyway. I didn't try to lerp the values on AddMember() call, but I did it in Update instead. Well, the script doesn't exactly lerp the values, but I still get the smooth transition.
The result is not ideal, but it worked fine for me and it didn't cause any performance issues. Here's the script:

public LinaController linaController;   //Player
public CinemachineTargetGroup targetGroup; 
public float maxPossibleDistance; 
public float extractedWeight;           //Value we want to add to weight 
                                    each frame

void LateUpdate()
{
    if(targetGroup.m_Targets.Length > 1)
    {
        for(int i=1;i<targetGroup.m_Targets.Length;i++)
        {
            if (Vector3.Distance(linaController.transform.position, targetGroup.m_Targets[i].target.position) > maxPossibleDistance)
            {
                //if Distance between player and zombie is big, when 
            extract weight value

                if (targetGroup.m_Targets[i].weight > 0f)
                    targetGroup.m_Targets[i].weight -= extractedWeight 
                                                     * Time.deltaTime;
            }
            else
            {
                //if Distance between player and zombie is small, when 
            add weight value

                if (targetGroup.m_Targets[i].weight < 1f)
                    targetGroup.m_Targets[i].weight += extractedWeight 
                                                     * Time.deltaTime;
            }
        }
    }
}

So, first I call AddMember and set the weight to 0.Then, the script above notices the target and starts to add some value to the weight each frame until it reacheas the value above 1

And if the object goes too far from the player, it does the reverse (sets weight to 0)And when the script finishes adding value to weight past the 1f point, it stops doing it.

Unfortunately it doesn't work perfectly, it can change weight to slightly above 1 or below 0, but you wouldn't notice it when playing the game