[services] don't hardcode 'users' group for register_user
Let the caller choose which group the user should be in. Related to #3020639
Caveat: this is a possible security hole if untrusted values are allowed.
\.svn
^build$
^dist$
\.pyc$
\.pyo$
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\~$
\#.*?\#$
\.swp$
^doc/book/en/apidoc$
\.old$
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.*/data/database/.*\.sqlite
.*/data/database/.*\.config
.*/data/database/tmpdb.*
.*/data/ldapdb/.*
^doc/html/
^doc/doctrees/
^doc/book/en/devweb/js_api/
data/pgdb/