The simple answer here is yes! Groups are designed to be manually created chunks of data, whether these are created from sign up forms, dataloads or just by you clicking to add someone into them. You can add as many groups as you want and contacts can be in any number of groups, it's up to you how you want to break down your data and use it.
First, go to contacts and click on the contact you want. This will load up the contacts profile.
Once in this page, then scroll down to see the groups section within the contact. Click on the Group tab and use the drop down menu to see the available. Simply select the groups you want and click on the add to group button.
That's it! It's done. You have added your contact to more than one group.