My wife and I pulled off a tag-long trip just this past February, and we decided it only about 6 weeks before the Mother and Sister in law left for WDW. The plan was we were to arrive on a Sunday, and my Father in Law would join up with us on Wednesday afternoon. We had DVC suites (1 bedroom, kitchen), but took along an inflatable mattress for when the Father in Law arrived.
The four of us were going to the PPP on the Monday evening, and all four of us had the best time for that day!! All four of us were laughing and having a good time like we were all 7 years old, and the youngest in our group was 18. We even coaxed our mother in law onto Goofy's Barnstormer, and she is scared of rollercoasters... so this was a big deal.
On the tuesday after relaxing (the girls had a spa day at Saratoga, and I hit the gym and relaxed by the pool) at Saratoga, we had a surprise arrive at the room, the Father in Law decided he didn't want to stay at home in the cold and work anymore, he decided to rearrange his plans and arrived a full day early. We had the best of times the rest of the trip, we still make jokes and recall this trip as one of the best ones to date!