In Contacts, click on Import Contacts.
This allows you to import contacts in bulk. You then have the option to add these contacts to an existing or new group. The contacts must be saved in a comma delimited CSV file (you can choose this as a "Save As" option in Excel).
Select the file
You'll be redirected to a new page where you'll be able to browse and select the file you wish to upload.
For more info on the import file requirements, check this article.
Assigning contacts to groups
You have two options (which you can check simultaneously).
Create New Group(s) will create the contact group and assign the contacts to this new group. The group email alias will be the group name without spaces (in this example, MyNewGroup).
You can also add contacts to existing contact groups. Just start typing and the list of groups will appear. You can select as many groups as you wish.
If you don't select any of these options, all uploaded contacts will be assigned to the All Contacts Group.
Once you are ready to go, click on Import Contacts.
If you upload more than 5,000 contacts (or a file larger than 50MB), SMS Central will do the processing in the background and let you know when it's ready.