Adding a SP.FieldUserValue into the Visitor/Member/Owner group of a site

Question

I'm creating a website based on what a user submits in a form
3 fields are reserved for the 3 types of default groups that you're asked for when creating a subsite manually :
- Owner
- Members
- Visitors

I noticed that these groups aren't created when creating the site via JavaScript
So I created them manually, after retrieving the field's value containing the users(requestData is a list line and value is the field internalName) :

SetupGroup(value, suffix)
{
    var users = this.requestData.get_item(value);

    var groupInfo = new SP.GroupCreationInformation();
    groupInfo.set_title(this.subweb.get_title() + ' - ' + suffix);
    groupInfo.set_description('Ca marche presque correctement !');

    var newGroup = this.subweb.get_siteGroups().add(groupInfo);

    users.forEach((user)=>{
        newGroup.get_users().add(user);
    });

    return newGroup;
}

But the content of the field is SP.FieldUserValue instead of SP.User and i get the error The parameter loginName cannot be empty or bigger than 251 characters once i try to throw a query to save this
How can I achieve it ?

Also, when the group is created, do i have to simply replace the Associated groups as such, or do I have to give the permissions to each groups myself to get the same behavior as a manually created site :

this.subweb.set_associatedVisitorGroup(this.SetupGroup('Visiteurs_x0020__x0028_Lecture_x'), 'Membres');

(SetupGroup returns a group)

Answer 1

Finally found out

Here's the code, users is a collection of SP.FieldUserValue :

AddUsersToGroup(users, group)
{
    users.forEach((user, iteration)=>{

        var userID = user.get_lookupId();
        var userInfo = this.web.get_siteUserInfoList().getItemById(userID);

        this.context.load(userInfo);
        this.context.executeQueryAsync(()=>{
            var newUser = this.web.ensureUser(userInfo.get_item('Name'));
            group.get_users().addUser(newUser);

            this.context.load(group);
            this.context.executeQueryAsync(()=>{
                console.log('User added : ' + userInfo.get_item('Title'));
            }, QueryError);
        }, QueryError);
    });
}

This will just add the user list to a given group, now if you want to give these groups the same rights as the Visitor/Member/Owner group, use the following guide : https://msdn.microsoft.com/en-us/library/office/hh185014%28v=office.14%29.aspx?f=255&MSPPError=-2147217396
And apply the roles :

• SP.RoleType.administrator for owners
• SP.RoleType.contributor for members
• SP.RoleType.reader for visitors