You can also use a query in SQL Management Studio to add new users. This query works for NAV 2016 database schema (I haven't tested it in NAV 2015, but it probably will work). Note that this query must be executed one time per user, this query works only for Windows users, and you would need to change some of the parameters to fit your particular situation. I highlighted the parameters in bold. The first parameter is the database name. The other parameters in bold may be self-explanatory.
USE [NAVDEMO]
DECLARE @USERSID uniqueidentifier, @WINDOWSSID nvarchar(119), @USERNAME nvarchar(50), @USERSIDTXT varchar(50), @EMAIL varchar(50)
SELECT NEWID()
SET @USERNAME = 'WIN-V5E6TMK7C4L\Administrator'
SET @USERSID = NEWID()
SET @USERSIDTXT = CONVERT(VARCHAR(50), @USERSID)
SET @WINDOWSSID = 'S-1-5-21-3993832035-3231277643-1611983836-500'
SET @EMAIL = 'email@email.com'
INSERT INTO [dbo].[User]
([User Security ID],[User Name],[Full Name],[State],[Expiry Date],[Windows Security ID],[Change Password],[License Type],[Authentication Email],[Contact Email])
VALUES
(@USERSID,@USERNAME,'',0,'1753-01-01 00:00:00.000',@WINDOWSSID,0,0,@EMAIL,@EMAIL)
INSERT INTO [dbo].[User Property]
([User Security ID],[Password],[Name Identifier],[Authentication Key],[WebServices Key],[WebServices Key Expiry Date],
[Authentication Object ID])
VALUES
(@USERSID,'','','','','1753-01-01 00:00:00.000','')
INSERT INTO [dbo].[Access Control]
([User Security ID],[Role ID],[Company Name],[Scope],[App ID])
VALUES
(@USERSID,'SUPER','','0','00000000-0000-0000-0000-000000000000')
INSERT INTO [dbo].[User Personalization]
([User SID],[Profile ID],[Language ID],[Company],[Time Zone],[Debugger Break On Error],[Debugger Break On Rec Changes],[Debugger Skip System Triggers],[Locale ID])
VALUES
(@USERSID,'SALES ORDER PROCESSOR','1107,'Cronus Australia Pty. Ltd.','FLE Standard Time','1','0','1','1033')
GO
