You can manage and modify users using Sitefinity API. You can find detailed information about this in the following link from our documentation.
You can also check the articles from the link below which could be very helpful to get familiar with the permissions and roles in Sitefinity.
Users, roles, and permissions
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items