Can anyone stay at Amritapuri, and do I need to register first?

Yes. According to the official FAQ, everyone is welcome to stay at any time and take part in the daily activities. International visitors are asked to confirm their arrival date in advance by registering online. Indian visitors are asked to email [email protected]. Groups of six or more should give at least 7 days' notice. Amma travels often, so check her schedule before you plan.