You can set default role for a new team member by configuring your SAML identity provider with the following attribute:
Attribute | SAML Name | Allowed values |
User's Role | urn:oid:1.3.6.1.4.1.5923.1.1.1.1 | Admin, User, Readonly or Restricted |
Note: The attribute Restricted is for the role "User with link-level permissions"
You can find more information on SSO and Teams the following articles :