Assuming you mean you want a textbox which contains two or more names and you want to search some database for one or more of the names...
It's not quite that simple. First, you need your user to indicate where a name ends: so get him to type commas or semicolons for example:
dave, pete, mike
Then assuming you are talking about SQL, you need to convert this to a format that SQL can accept:
string s = "dave, pete, mike";
string[] names = s.Split(',').Select(n => string.Format("'{0}'", n.Trim())).ToArray();
string sql = string.Format("SELECT * FROM MyTables WHERE username IN ({0})", string.Join(",", names));
But...That is pretty dangerous - it leaves you wide open to SQL Injection attack, so you will probably have to sanitize your user input.