user table join another table

If BusinessUnit is a custom module and assuming that the foreign key in the Users table and the primary key in the BusinessUnit table are both named BusinessUnitID, this snippet should return a dataset of the inner join of the two tables:

var dataSet = UserInfoProvider.GetUsers()
    .Source(source => source.Join<BusinessUnitInfo>("BusinessUnitID", "BusinessUnitID"))
    .Where(whereCondition)
    .Result; //dataset with properties of both tables, take this out if you want just the collection of UserInfo

If BusinessUnit is instead a custom table, then you can use ConnectionHelper to execute a SQL query, like so:

string selectStatement = 
    "select u.*, b.* from CMS_User u inner join BusinessUnit b on u.BusinessUnitID = b.BusinessUnitID";
QueryDataParameters parameters = null;
var dataSet = ConnectionHelper.ExecuteQuery(selectStatement, parameters, QueryTypeEnum.SQLQuery);

Obviously, modify the select statement and parameters as needed. Refer to this documentation to see the overloads of the ConnectionHelper.ExecuteQuery() method.