Using roles for this requirement sounds like a good solution but by default WFFM Create User action does not support the Roles. You have more possibilities how/when to do this.

#1. Assign roles manually#

This is the simplest way. Just use User Manager and assign roles manually. Of course it is not really efficient to assign particular roles for every new registered user.

#2. Custom assign role WFFM action#

You can create your own save action and set it up on your form.
The implementation will look like something like this:

    using System.Web.Security;
    using Sitecore.Data;
    using Sitecore.Form.Submit;
    using Sitecore.WFFM.Abstractions.Actions;

    namespace MyProject.Feauture.Security.WFFM.Actions.CreateDisabledUser
    {
        public class AssignRoleAction : CreateUser
        {
            public override void Execute(ID formId, AdaptedResultList adaptedFields, ActionCallContext actionCallContext = null, params object[] data)
            {
                var userName = this.DomainField + "\\" + adaptedFields.GetValueByFieldID(ID.Parse(this.UserNameField));
                Roles.AddUserToRole(userName, "Your role name");
            }
        }
    }
#3. Assign the role on login#

So you can also assign the role on login, if it's not added to your user probably based on the domain.