I have a community with 100 Partners Users and I need to switch one or all of them to try lightning in that community but I don't know why I can't.
Lightning Experience (and Classic) are specific to Internal Users. You don't turn it on/off for Community Users.
Community Users get access to your Org only through Communities. And that Communities can be built based on different templates which drives the UI/UX for that Community. So as long as your Community is built providing a Lightning User Experience, they will get the look and feel based on that. If your Community say is built on Salesforce Tabs + Visualforce and they primarily reflect the old look and feel, then you will need to redesign your Community.