To change the status of a user go to Admin > Users and select a user from either the user list or with the search tool. Click on their name to access their user information. This view will allow you to update their status by selecting from the Status drop down list. You can also add and remove any groups a user should be associated with.
*Note that when suspended users are made active again they will automatically receive a Reset Password Email, even when welcome mails are not enabled.