Should former group member be included as a co-author in a follow-up to his work?
There are three distinct levels of credit that the former postdoc might be given, depending on their level of contribution to each particular new publication.  In order of increasing significance, these are:


Citation.  The postdoc has done significant prior work for these papers, and so clearly this prior work must be cited.
Acknowledgement.  If the postdoc contributed materially to the new paper, but not enough to merit authorship, then acknowledgement is the typical compromise "middle ground."
Authorship (though not in one of the "privileged positions" such as first author or last author, if they are held significant in the particular subfield).


The boundary between citation and acknowledgement is usually pretty straightforward: did the individual make any contribution that significantly aided the new work beyond providing nice, clear explanations of the content of their publications?  If not, then merely citing should be sufficient.

The boundary between acknowledgement and authorship is extremely field dependent, and moreover often affected strongly by the subfield.  With ethical and trusted colleagues, the best way to navigate this question is often simply to have an open discussion together about where and how to draw this boundary.