How can I implement MFA in Identity Center for a group/role?

Hello.

I don't think "aws:MultiFactorAuthPresent" can be used as a global condition key for policies in IAM Identity Center.
Therefore, you cannot enforce MFA by policy in the same way as you can with IAM users.
If you are using an external IdP, you may be able to use session tags to assess whether you are authenticated with MFA, as described in the following document:
https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_policies_condition-keys.html#condition-keys-multifactorauthpresent:~:text=%E2%80%93%20Single%2Dvalued-,aws%3AMultiFactorAuthPresent,-Use%20this%20key

This condition key is not present for federated identities or requests made using access keys to sign AWS CLI, AWS API, or AWS SDK requests. To learn more about adding MFA protection to API operations with temporary security credentials, see Secure API access with MFA.

To check whether MFA is used to validate IAM federated identities, you can pass the authentication method from your identity provider to AWS as a session tag. For details, see Pass session tags in AWS STS. To enforce MFA for IAM Identity Center identities, you can enable attributes for access control to pass a SAML assertion claim with the authentication method from your identity provider to IAM Identity Center.

It is possible to enforce MFA upon sign-in in the IAM Identity Center rather than enforcing MFA only for specific accounts.
https://docs.aws.amazon.com/singlesignon/latest/userguide/how-to-configure-mfa-device-enforcement.html