NASCAR's Competition Department distributes entry blanks for each race to owners in the series.
Those entries must be returned before the deadline, which is set 13 days in advance of the event.
When the entry is received by Competition, the car is added to the entry list.
There is an inspection fee for each entry.

What happens if all qualifying gets cancelled [rain/weather/other]?
If bad weather cancels qualifying, the final starting lineup will be determined by practice speeds. The same rule book procedures will be used to determine eligibility to start a race. If weather cancels practice sessions, then the starting lineup will be set by points, per the rule book.
IF QUALIFYING AND ALL PRACTICES ARE CANCELED, then:
After the 5th race of the season, the current season OWNERS points are used:
#1) - First 35 by current Owner Points [team does not have to attempt all the races]
#2) - Last Years Cup Champ or the Event Champion if not already in the field
#3) - Car Owners whose team has won in the previous and current season not already in the field
#4) - Drivers who have won in the previous and current season not already in the field
#5) - Past Sprint Cup Champions not in by 1 thru 4
#6) - Qualifying Attempts with Owners points as the tie breaker
#7) - Qualifying Order Position
(all above apply as long as the team was entered by the entry deadline - usually 7-13 days before the race)

If qualifying AND all practices are rained out or cancelled
the teams making the race would be:
with the #21 not running at Darlington, the #13 moves in to the Top 35 and a guaranteed starting spot
#71, 37th in Owners pts, 9 attempts
#37, 38th, 9
#38, 39th, 9
#46, 40th, 9
#66, 41st, 9
#87, 43rd, 9
#60, 45th, 9
#92, 47th, 5
missing the race would be:
#30, 46, 1
#50, none, 0
#75, none, 0
#81, none, 0

Top-35 in Owner Points and guaranteed a starting spot at Darlington:
Three teams are tied for 35th, #32, #13 & #71. The #32 gets the 35th with three best finishes of 15th, 22nd & 24th, the #13 is 36th with three best finishes of 15th, 22nd & 25th, the #71 is 37th with a best finish of 19th. With the #21 still in the top-35 [attempts do not matter if in the top-35] but skipping Darlington, the #13 will have a guaranteed spot at Darlington.
32) #21-Bayne/Wood Brothers, 140, +8
33) #7-RGordon/RGM, 137, +5
34) #36-Blaney/TBR, 136, +4
35) #32-Schrader/FAS Lane, 132, +0 point ahead of 36th
these teams / drivers would need to qualify by speed after race 5:
36) #13-Mears/Germain, 132, 0 points out of top 35
37) #71-Lally/TRG, 132, -0
38) #37-Raines/Front Row, 98, -34
39) #38-Kvapil/Front Row, 91, -41
The teams are NOT locked in past the next race as the owners points change after each race.
For the owner points standings, see my Owners Points Standings page.(4-30-2011)

Past Champions Provisional Berths in Sprint Cup: A past champion’s provisional may be used by an eligible driver a maximum of six times over the course of the NASCAR Sprint Cup Series season. A team with a past champion eligible driver may only use this provisional a maximum of six times during a season. The provisional gives the eligible driver the 43rd and final starting position in the race field. This is only for a team/driver OUTSIDE the top-35 in owners points, drivers whose team is in the top-35 in owners points, would already be guaranteed in the field and not need the Past Champions Provisional
See my 2011 Past Champions Provisional Page for more.