There are details about how to limit invites to staff members in the “Only allowing staff to send invites” section of this topic: How to send, configure, and accept Discourse invitation emails.
The easiest way to limit sending invites to staff members is to enable the
must approve userssite setting. When enabled, this setting prevents regular users from sending invites, but allows staff users to send invites. Users invited by staff are automatically approved.If enabling
must approve usersis not an option for your site, invites can be limited to staff members by setting themax invites per daysite setting to 0. This setting doesn’t apply to staff members, so they will be able to send an unlimited number of invites.