Then can they choose who can join their forum based on criteria of their choosing.
Not necessarily if that involves their personal data
Article 21 (2) GDPR states
"Where personal data are processed for
direct marketing purposes, the data subject shall have the right to
object at any time to processing of personal data concerning him or her for such marketing, which includes profiling to the extent that it is related to such direct marketing. "
So while it might be posible to force newsletter upon registration (I don't know if that is the case), users could always object to usage of their email address for marketing newsletters.
You could then probably ordinarily terminate their usage contract within the time agreed to in your terms of service, however you can't terminate it extraordinarily becuse of their objection.
You also can't deny them to make a new contract afterwards, eg. to sign up for a new account.
So my interpretation in a nutshell:
You can't force users to accept newsletters used for commercial advertisting/marketing.
If you only use them for real news (like announcing changed Terms of Service/Privacy Policy, Maintenance, etc.) that might be a completely different story.