Fix/return value of group add user

[marc-farre]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the develop branch, not the master branch if no hotfix
• When resolving a specific issue, it's referenced in the PR's description (e.g. Fix #xxx[,#xxx], where "xxx" is the Github issue number)
• All tests are passing
• New/updated tests are included
• Changelog was modified

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[what-the-diff]

PR Summary

• Fixes issue Fix/return value of group add user #6875 related to user group addition
  The updates in this Pull Request address a challenge where the method of adding a user into a group was giving incorrect feedback. Specifically, even when the user was successfully introduced, it was reporting a failure if the current user was not logged in. The implemented fix rectifies the issue by checking if the user is a guest before generating a response.

• Notification improvements for guest users
  An adjustment has been made to the method of adding users into groups. The change ensures that notifications are not sent out unnecessarily when the individual added to the group is a guest. This results in a more efficient and less intrusive notification system.

• Code style enhancement within the group user approval notification
  There's been a subtle but crucial enhancement regarding the formatting of the code. Specifically, the indentation within the translation strings has been aligned in order to adhere to the overall style of the project's coding. This improves readability and maintains code consistency throughout the project.

[marc-farre]

@luke- small fix:

On Group::addUser(), if the current user is not logged in (console command or user registration), the method returns false even if the user was added to the group