Should I link my groups together?
If you have multiple groups, which have members from a common set of people, then those groups should probably be linked.
If I change my mind can I unlink my groups?
Yes. You can link and unlink groups (which you manage) at any time.
Does this affect how my groups are billed?
No. All groups on Gaggle Mail are billed completely separately, even if groups are linked you will have a subscription for each group based on the number or members in that group.