If you mean that you would like to change the distribution group's password.
Because there is no password for a distribution group, so it's impossible to change the distribution group's password.
According to your case description, let me verify the issue you meet is that you don't want users to be able to join groups by themselves, right? Please make sure that my understanding is correct. If in this case.
You could set the distribution group’s permission to closed. Then the members can be added only by the group owners.
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