To add a contact/company to a group, simply go to the contact’s profile in your CRM and click “Group” in the “Attach an item to this company/contact” section in the left-hand column. In the window that pops up, click the name of the group in which this contact/company belongs, and the contact will be added. To add a contact/company to multiple groups at the same time, simply tick the boxes to the left of each desired group and select "Add to group." You can also create a new group on the fly from this window using the text box at the bottom.
To remove a user from the group, simply click the "X" in the group label that appears near the top of a contact's profile.