Thank you for your post.
I would create a table with two text fields:
This table would have all people with login names.
Then, in your other table, create a relationship between the current table and the newly created table, where the "key" field would be the Login Name. Then, when you enter a login name, it automatically links to the appropriate record in the newly created table and displays the name.
On your layout, you can use the Field tool to display the "Name" field from the related table.
If you need further clarification, please let me know.