If you're an admin, go to the admin interface, projects. There you'll see the field "Only users in the following groups can see this project". Select the appropriate groups. And then assign users to the relevant groups. You can of course create any number of groups in the group admin section.
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How do I "Restrict projects to certain groups of users" as stated on http://phpbt.sourceforge.net/ I cannot find any documentation on this feature.
If you're an admin, go to the admin interface, projects. There you'll see the field "Only users in the following groups can see this project". Select the appropriate groups. And then assign users to the relevant groups. You can of course create any number of groups in the group admin section.