In the thread "Send email to LimeSurvey Users" the recommendation was to create a group and add all users to it, then email the group. ("Users" here means "people who create surveys", not respondents)
Creating the group is easy, but I see no way to add all my users to it: the "add user" facility in a group only lets you add them one by one, and I have dozens of them.
I think your only chance is to do it one by one. Usually you add people to a group when creating the user and in general the number of users of the LS backend is not soo elevated. Which is probably why there is no feature for bulk adding.
Maybe you can do something with the LS API, but with "only" dozens, you are maybe quicker adding them one by one than to find out if it is possible and add the respective API call. But I am no expert on this and I don't know if the API allows this.
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Note: I answer at this forum in my spare time, I'm not a LimeSurvey GmbH employee.
So, if you've got the coding skills and if you also have to add multiple users in the future, the API is probably the easiest way to go. Otherwise, like holch mentioned, manually adding the users.