You can update your query to look like this:
SELECT clients.ClientID, clients.ClientUserID, clients.Title, clients.FirstName, clients.LastName, clients.DateOfBirth, clients.Email, clients.Address, clients.Address2, clients.City, clients.State, clients.Zip, clients.Country, user.*
JOIN user ON clients.ClientUserID = user.userid
WHERE ClientID = ParamClientID OR ( -1= ParamClientID2 AND ClientID= ParamSessionClientID)
It is just the addition of the join statement that joins the table, you will need to update for the name of the user table, it is just user in the query, also you need to update the userID column to match the name of the column in your user table.
Using joins can be very helpful when you are trying to get more related information from a recordset. One site that I use as a good reference for the SQL is w3schools.com, here is a link to their join section:
This section and the others for sql have some very good information in them and can help you learn how to craft more advanced queries.