I don’t know if there is an place in GoogleGroups user interface to achieve this, but i did not find it. But you can subscribe with every EMail address to a GoogleGroups when sending a mail to the address of the group and add an “-subscribe” before the @. An Example:
You can subscribe to the group TiddlyWiki@googlegroups.com when you sending a mail to TiddlyWikifirstname.lastname@example.org . I ever give this message the subject “subscribe” but i don’t know if important.