question

When a player joins I want them to be able to enter a guild name and if that guild name does not exist it will create one else they will automatically join that guild.

If you mean when a player creates account, they can also enter a guild name. And after account created, they will automatically join that guild, or create with that name if it doesn’t exist. You can use Groups, Guilds and Clans - PlayFab | Microsoft Learn to do it. Steps are follows, use Groups - Get Group - REST API (PlayFab Groups) | Microsoft Learn to check if the group exist, if it does, call Groups - Add Members - REST API (PlayFab Groups) | Microsoft Learn to add this player to the group, you may use PlayFab CloudScript using Azure Functions - PlayFab | Microsoft Learn to do this step as this API requires a title level token. If it doesn’t, call Groups - Create Group - REST API (PlayFab Groups) | Microsoft Learn to create one.

I also want a statistical leaderboard that shows an accumulation of a custom score each player in that guild obtained within a set amount of time.

Group leaderboard is not supported currently, you can implement it using an external database like Redis or wait for Leaderboards v2 - PlayFab | Microsoft Learn.