Google has excellent documentation here for viewing, grouping and sharing your contacts.
This documentation describes how you can create a label (also known as a mailing list, or contact group) and populate it so that you can send to a group of people by just typing the name of the label. For example, you might be on a search committee or you'd like to email members of your department, so you could create a label, give it a name like 'Search' and add the members of the committee. When you'd like to email all the committee members, you can click on Compose, type in Search, and you now have an email that goes to all of the members.
You can always go in and edit your group's membership if you'd like to add or delete a member.
Contact groups cannot be shared without sharing all of your contacts with someone else. If you have a need for a shared contact group, creating and managing a Google Group gives you many more options. Most Google Groups are set up so that all members can post to the whole group. Managers of the group can add and remove members so that each individual member of the group doesn't have to maintain their own contact group membership.
Google Groups can be requested using this form found under Forms and Policies in myBucknell.