George,
there is a nice discussion about group currently.
You should have a look to it.
For your case, the group is not mandatory.
The loadbalancing function is configured with owner and content.
A service can be part of multiple content.
So what you need is this
service A
service B
service C
owner MyCompany
content web
add service A
add service B
content mail
add service B
add service C
The group is only required if you want to do source nating which normally is not needed.
Again, read the other topic in this forum talking about groups if you need more info.
Gilles.