Вы можете попробовать это.
DECLARE @UserAccess TABLE (IPAddress VARCHAR(16), UserName VARCHAR(20), [DateTime] DATETIME)
INSERT INTO @UserAccess VALUES
('198.168.1.101', 'User 1', '2019-01-01 9:00:00'),
('198.168.1.101', 'User 2', '2019-01-01 9:00:05'),
('198.168.1.101', 'User 3', '2019-01-01 9:00:10'),
('198.168.1.101', 'User 4', '2019-01-01 9:00:15'),
('198.168.1.101', 'User 5', '2019-01-01 10:00:00')
SELECT IPAddress, UserName, [DateTime] FROM @UserAccess U1
CROSS APPLY (SELECT COUNT(*) CNT FROM @UserAccess U2 WHERE
U1.IPAddress = U2.IPAddress
AND ( U2.[DateTime] BETWEEN DATEADD(SECOND,-300, U1.[DateTime])
AND DATEADD(SECOND, 300, U1.[DateTime])) ) X
WHERE X.CNT > 1
РЕЗУЛЬТАТ
IPAddress UserName DateTime
---------------- -------------------- -----------------------
198.168.1.101 User 1 2019-01-01 09:00:00.000
198.168.1.101 User 2 2019-01-01 09:00:05.000
198.168.1.101 User 3 2019-01-01 09:00:10.000
198.168.1.101 User 4 2019-01-01 09:00:15.000