Can you explain the relationship between groups, i.e a parent group
If your monday weeders group belongs to the volunteers group, then members of the monday weeders will be acted upon automatically as members of the volunteers group.  So when you send an email to the parent group of volunteers it will be sent to all the members of monday weeders and any other group that's a sub-group of volunteers.

Is that what you're wondering?

There's more information about groups here https://docs.civicrm.org/user/en/latest/organising-your-data/groups-and-tags/.