The predefined membership types are specified in the membershipType field of the OrganizationConfig plugin parameter.
See 02portal.war:/WEB-INF/conf/organization/organization-configuration.xml for the full content.
<field name="membershipType">
<collection type="java.util.ArrayList">
<value>
<object type="org.exoplatform.services.organization.OrganizationConfig$MembershipType">
<field name="type">
<string>member</string>
</field>
<field name="description">
<string>member membership type</string>
</field>
</object>
</value>
<value>
<object type="org.exoplatform.services.organization.OrganizationConfig$MembershipType">
<field name="type">
<string>owner</string>
</field>
<field name="description">
<string>owner membership type</string>
</field>
</object>
</value>
<value>
<object type="org.exoplatform.services.organization.OrganizationConfig$MembershipType">
<field name="type">
<string>validator</string>
</field>
<field name="description">
<string>validator membership type</string>
</field>
</object>
</value>
</collection>
</field>