Since Portal for ArcGIS version 10.4.1, it is possible to specify a custom role as the default role for newly added members.
However, in Portal for ArcGIS version 10.2 and 10.3, it is not possible to set a custom role as the default role for the new users. If the portal site is configured to use automatic registration, enterprise users who access the site for the first time are automatically added as members. By default, these members are assigned the 'User' role.
In Portal for ArcGIS 10.3.1, the default role can only be specified for new enterprise users only if they log in via Security Assertion Markup Language (SAML).
The privileges set for a member with the 'User' role includes using and sharing maps and applications, creating new content, editing features, as well as joining and creating groups. However, some organizations may want new users to only have certain custom-defined roles that differ from the privileges defined for the User role (for example, limiting privileges for users to only view the maps and not add new content). The administrator of the portal site can change the default User role by configuring custom roles for members after their accounts are added.