Create a text field, there are auto-entry options for both username (the name of the user's computer) and accountname (the name of the logged in filemaker account).
There are creation and modification options for each.
If you need a running history of modifications, and not just the most recent modifier, then you need to create what's called an audit log. There are existing products for filemaker developers that handle audit logging. Most notably and my favorite is UltraLog:
double click the field you just created, or click the "options" button to the right of the field name when it's selected.
This is important, basic stuff. Screenshot attached.