To create a User, e.g. a committee member, select Organisation > Users and Roles
If the Contact is already in the Users list, you can adjust their User Rights by selecting Edit User Rights and clicking on the appropriate check box item to unlock it - or, give them the permissions for that level.
If the Contact isn't yet a User, you can add them and give the appropriate User Rights.
Sessions User Rights
If a Contact has Sessions User Rights, when logged in they will be able to see their Dashboard, Sessions and their own Tasks.
Contacts with Sessions User Rights would generally be able to view all Sessions and create new Sessions.