User are organized in user groups. An user belongs to any at least to one of these groups. User is a entity with these properties: name, short name, phone, email, password and note. User can also have it's own reports, graphs, queries and notification. If the user belogs to more user groups, it gets the permision from them all.
User groups are mainly used for permision definition. For example internal users can edit and see all fields.
The external users on other hand can see (edit) only a range of fields. User groups can also define
shared reports, graphs,
forms, queries and
At last they are used in Choice list for reducing elements visibility.