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Modules for XOOPS use the XOOPS authentication so that all users will have access to the modules you choose as defined by your permission settings. In short, if you are using a module desiged for use for XOOPS, then you will not need to worry about separe user names and passwords as all of these are integrated in the system. I just hope I am understanding the question clearly.
If you are using a separate script that is not associated with XOOPS, you would have to modify that external scripts SQL queries to look in your XOOPS environment tables for user amd group perms, which may or may not be worth the trouble.
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