[MacPorts] #38440: document add_users

[MacPorts]

#38440: document add_users
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  Reporter:  aronnax@…    |      Owner:  markd@…
      Type:  enhancement  |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  guide        |    Version:  2.1.3
Resolution:               |   Keywords:
      Port:               |
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Comment (by egall@…):

 I think this might be related to ad/or a duplicate of #16900

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Ticket URL: <https://trac.macports.org/ticket/38440#comment:3>
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