Auto add new users also as users of another blog

Hi,
I have a support blog (support.domain.com) set up separate from the main blog (domain.com). Is there a plugin or way to automatically make new blog users also users of the support blog?

I set it up this way as to not over tax the main blog and also if the support blog crashes it won't affect the main blog. Is this correct thinking?

I may also use the Forums plugin on the support blog. Would an auto add users like above make all global users automatically members of the forum?
Thanks,
ChuckS