The information is in the TD Support area of the USCF website. See their FAQ at https://secure2.uschess.org/TD_Affil/faq.php . The relevant section reads:
I have a non-member in my tournament. How do I do a tournament membership?
First, does this player have a USCF ID yet? If not, see the answer to the previous question (dealing with a player who does not have an ID yet) and follow the first step to get a USCF ID for this person.
Once you have an ID, you can use it in the rating report. When you validate the rating report, this person will be flagged as a non-member, causing an error.
Players who are under 25 qualify for the a $7 Junior Tournament Membership, all others qualify for the $12 Adult Tournament Membership. $5 of a $7 Junior TM fee and $10 of a $12 Adult TM fee can be applied towards a USCF membership if purchased within 60 days. If at all possible, please supply us with an email address for Tournament Members so that we can notify them and give them a chance to apply that credit towards a USCF membership.)
When you go to edit the rating report, there should be a checkbox to purchase a tournament membership for this player. (Tournament Memberships are not applicable to USCF National Events.)
Once you check the tournament membership box, have the rating report checked for errors again.
The tournament membership fee will be added in to the total fee when you submit the rating report, along with the rating report fee and any other fees, such as Enhanced Grand Prix fees or FIDE fees.
Please note that the member must already have a USCF ID, if there is no USCF ID associated with that member this will generate an error that must be corrected before the Tournament Membership box will be displayed.
Also see Tournament Membership Program.
At an upcoming tournament, (Shameless plug time: October 2, Auburn Hills, MI. http://gamesinmichigan.com/chessforcharity) I intended to offer unrated players the option of playing in a rated section by purchasing a single event, or tournament, membership from the USCF. However, I can't find any information about doing this on the USCF site. I know the option used to be available. Does anyone have any information on how to do this or how much it costs?