You can create groups of people in your Address Book section of the Account Center.
If you do this, you can later add that group to a meeting by opening the "Address Book" on the Invite Participants screen.
When you are creating any new meeting from the "Schedule" section, you will get an option "Invite participants".
Check this box to make the field to add participants appear.
Click onto the user icon in the email field, it will give you list of all your participants that you have added in your address book. You will see this list in alphabetical order.
In this section you will get options to sort the participants list as per your convenience i.e "All", "Company", "By group" and "By list".
Here you have to sort this list "By group" so that you can view all your participants sorted in your various groups.
You need to check the box infront of the name of the group, so that the whole group can be selected and invited to the session.