You can create a contact group from an email you have sent or received by copying the email addresses in the To or Cc box and pasting these into a new contact group.
Step by step instructions are available from Microsoft.
Think privacy...when naming your contact group, think about making sure it has a name that is obviously different from any other contact lists or resource accounts you use. This is essential to avoid inadvertently sending information to the wrong person / group. The most important thing is that the first few letters are not the same as any other email addresses or groups you use. For example, rather than having the groups “Childhood Yr1” and “Childhood Yr2”, perhaps call them “1st Year Childhood” and “2nd Year Childhood”, so that the first few letters are different. The autocomplete feature in Outlook can be very helpful but only if it autocompletes with the correct address.