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Yes, with the correct permission settings. You can read more about it under the LDAP user group settings.
How many more users can we add above our current license tier?
It depends on the company processes. How often does your users use Jira? Do they just check it several times a day/week for a short time or do they need it constantly?
You can find out more information about your Jira usage with our License Monitoring application.
Nevertheless, we don’t recomment to exceed your current license tier more than 50% (10 user tier → 15 users, 100 user tier → 150 users, etc.). Above that percentage, you may need to add a lower Treshold setting that could disturb your users.