[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
 Keywords:               |       Port:
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 The add_users command is not documented in the Guide; only the older
 adduser and addgroup are.

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