I have a dinner reservation for my dd16 to celebrate her birthday. Our trip was all planned out for the 2 of us. Well 60 days out she decides she wants her brother to come on the trip. There are no ADRs for 3 on the night of our dinner. Does anyone know if he can join us when we arrive for dinner?
1) You cannot simply add another person, or more, to an existing reservation.
2) The reservation must be new.
. . . the old ressie is cancelled
. . . the new ressie is made for the number of folks you want
. . . if you or the CM mess-up and cancel first, then you might not even be able to get the new reservation
. . . lots of people have literally been not able to get back into their eatery
3) It is better to try for a single person ressie, as ADR's for BoG are hard to get.
4) If not able to get the ressie, someone is going to stand outside and wait while you eat.
5) Basically, make the initial ressie for those you really want.
6) All too often, especially at popular eateries (like BoG), adding people is not possible.
7) See the notes below regarding extra people.
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THE QUESTION OF AN EXTRA PERSON or EXTRA PEOPLE (the "411"):
1) Disney is getting wise to the "but, they decided to join us" or "I miscounted".
2) People can't always get the ADR they want.
3) So, they under-count to get a table.
4) Then, they show up with "unexpected" additional people.
5) In a team meeting, CM's have been told to warn guests against this.
6)
If there is room, WDW will not turn away revenue.
7)
But, if busy, they will turn away the additions.
8) Go ahead and arrive with the extras, if you like.
9) Just be prepared to have someone stand around OUTSIDE while you eat.
10) I have heard explanations such as . . .
. . . "but, Aunt Suzy decided to join us and we need an extra seat"
. . . "but, two friends from off-site decided to join us and we need two extra seats"
. . . "but, brother-in-law decided to join us and we need an extra seat"
. . . "but, the entire Boy Scout Troop decided to join us and we need 14 extra sets"
. . . "but, my baby is so small that I didn't count her in the number of eaters"
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NOTE1: It is far easier going from an odd-number to an even number, such as from 7-people to 8-people, adding a person to an even number of diners, such as going from 6-people to 7-people. Adding odd person can mean adding a table, and this might not be possible. Going from an odd-to-even might only mean adding a chair or using an empty seat. So, if going from even-to-odd, you might not get the extra person a seat. It is always better to get another ADR with the proper people-count.
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NOTE2: Frequently a party-of-two will try to add a third person at the podium. This can be risky, because many eateries do have tables-for-two. If the guest was slated for one of these, there simply might not be a table available to seat the third person - even if an infant. The extra chair location might extend into an aisle and block access for other guests or Servers.
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NOTE3: Sometimes it is a threesome adding a fourth. Unfortunately, there are many eateries that do have 3-top tables - usually against a wall or pillar. They can't add a fourth chair, since that would invade the aisle and be unsafe for SERVERS.
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NOTE4: The final question is that of how many people are in the eatery. There are Fire Codes which dictate the maximum amount of people. If the capacity is exceeded, there are some big fines possible. And, Disney will not violate the max-capacity of an eatery. 
