Cannot see friends in Pokemon Sun/Moon Festival
Solution 1:
If your guest list is full, your friend will automatically be kicked out (disappear). The only way to fix this seems to be adding them to VIP lists which guarantees they'll be there every time you enter Festival Plaza.
However, this assumes that you're already able to see them - to make them appear for the first time you'll just have to refresh your guest list until they appear. To force a re-sync of the guest list, I'd recommend both parties to disconnect and reconnect to the Festival Plaza.
Source: Reddit & personal experience