question

Since you want to list all the groups associated with the player, then the GroupsModel.EntityKey used in PlayFabGroupsApi.ListMembership method is exactly the same entity key returned in LoginResult. What you need to do here is manually wrap it into GroupsModel.EntityKey like the following:

//TitlePlayerEntityKey is the property that stores entitykey returned by LoginResult.EntityToken.Entity
PlayFabGroupsAPI.ListMembership(new PlayFab.GroupsModels.ListMembershipRequest { Entity = new PlayFab.GroupsModels.EntityKey { Id = TitlePlayerEntityKey.Id, Type = TitlePlayerEntityKey.Type } },
    result =>
    {
        foreach (var group in result.Groups)
        {
            Debug.Log(group.Group.Type + " : " + group.Group.Id + " - " + group.GroupName);
        }
    },
    error => { Debug.LogError(error.GenerateErrorReport()); });