You can create a campaign from an existing group in the Groups 
tab.
In Groups
- Click on the Groups tab, on the left side of your screen.
- Click on the check box of the group for which you wish to send a campaign to. You can select as many groups as you want.
- Click on Create campaign.
- Enter the name of you campaign.
- Click on Create campaign. You will be redirected in the steps to create your campaign.
In a specific group
- Click on the Groups tab, on the left side of your screen.
- Click on the specific group for which you wish to send a campaign to.
- Click on Create campaign.
- Enter the name of you campaign.
- Click on Create campaign. You will be redirected in the steps to create your campaign.