welcome to Tiki!
In Tiki permissions are assigned to groups so you need to create the Groups for these users and assign it to them. Then each category can have the permissions assigned for any of those groups.
See https://doc.tiki.org/Groups-Management#Creating_Groups and https://doc.tiki.org/Groups-Management#Permissions_for_individual_users
If you need for each user his/her own group I recommend to enable the feature called "Create a new group for each user" on
tiki-admin.php?page=login under the "Other login settings" on the first tab.