Assume that you create a few groups and put some contacts in them in Microsoft Skype for Business 2015 (Lync 2013). Then, you enable the "Show contacts with away, offline and unknown presence in a separate group" option. After you exit and then restart the client, the groups don't contain all the contacts that belong to them. In the worst case, the groups may be empty. They'll only be filled up with the complete list of contacts while you scroll through the contact list.
To fix this issue, install the December 6, 2016, update (KB3127976) for Skype for Business 2015 (Lync 2013).
Note Lync 2013 was upgraded to Skype for Business in April 2015.