[Fixes #5801] Check User permissions for private group

[afabiani]

Checklist

Reviewing is a process done by project maintainers, mostly on a volunteer basis. We try to keep the overhead as small as possible and appreciate if you help us to do so by completing the following items. Feel free to ask in a comment if you have troubles with any of them.

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[t-book]

thanks @afabiani looks good. I can test/checkout the PR tomorrow morning.

[afabiani]

@t-book I'm going to add some test cases before considering this done

[codecov]

Codecov Report

Merging #6643 (bad3c87) into master (e1d986e) will increase coverage by 1.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6643      +/-   ##
==========================================
+ Coverage   57.99%   59.05%   +1.06%     
==========================================
  Files         188      188              
  Lines       21421    21427       +6     
  Branches     3574     3577       +3     
==========================================
+ Hits        12423    12654     +231     
+ Misses       7915     7653     -262     
- Partials     1083     1120      +37     

[t-book]

@afabiani I've tested this PR and it works great! Thanks!