The Set Up
If you want to play Dominion Prosperity with more than 4 players, we recommend increasing the number of Colonies as well: 15 Colonies with 5 players and 18 Colonies with 6 players. This isn’t specified in the rules of Intrigue, but we think that’s only because Intrigue came out before Prosperity.
Dominion Prosperity comes with 12 Colonies only, so you can use blank cards to replace the lacking Colonies.
Other than that all expansions can work well with 5-6 players, so we recommend you look at our Dominion expansion ranking and find the one that appeals to you most.