Go to Slack.
Scroll down to the Direct Messages section.
Click "+ Invite people".
In the pop-up window:
Enter the email address of the person you want to invite.
Select their role.
(Optional) Add them to specific team channels.
Click the Send button.
Please, note that since OrgaNice is still synchronizing users from Slack and doesn’t yet support adding or removing people directly, all user management must be done in Slack for now.