Here’s the quick version:
Top left corner, where it says “MAIL” with the down arrow.
Click the arrow, select CONTACTS.
New Group [left side]. Give it a name.
Click on the group name in the list [on the left].
Button up top – add names to the group.
You can manually type ’em in, copy/paste [I presume], etc.
Save the group.
To send to the group you type in the group name in the address field of the new message.
Want more details?
Creating Contact Groups – Gmail help
Quickly create large contact groups – Youtube video