Find all IP's or users who access a specific SQL Server
You can look at the currently running processes in SSMS in Activity Monitor. 

You can also get it using sys.dm_exec_sessions as well as sys.dm_exec_connections and sys.dm_exec_requests.

Query:

SELECT DB_NAME(database_id) as [DB]
    , login_name
    , nt_domain
    , nt_user_name
    , status
    , host_name
    , program_name
    , COUNT(*) AS [Connections]
FROM sys.dm_exec_sessions
WHERE database_id &gt; 0 -- OR 4 for user DBs
GROUP BY database_id, login_name, status, host_name, program_name, nt_domain, nt_user_name;


Hostname, process name or domain user names often give more information than just an IP address. Other columns can be added such as: login_time, last_successful_logon

IP and other information can be retrieved with a JOIN to sys.dm_exec_connections on session_id: client_net_address, local_net_address, connect_time, ports, ...

sys.dm_exec_requests can also give useful information: command type, sql_handle, ...

This query is just a sample. You must JOIN these 3 views together and output/store relevant information from either of them. 

Data from these view disapear when SQL Server is restarted. Therefore if the application is not regularly connected it may be a good idea to store them on a regular basis using a job or a script (Powershell).