The User service allows the management of the portal user and it's communities (aka Groups), Roles and UserGroups. It can be accessed through the static methods of UserServiceUtil. Following is a description of it's most important methods:

	public static com.liferay.portal.model.User addUser(
		java.lang.String companyId, boolean autoUserId,
		java.lang.String userId, boolean autoPassword,
		java.lang.String password1, java.lang.String password2,
		boolean passwordReset, java.lang.String emailAddress,
		java.util.Locale locale, java.lang.String firstName,
		java.lang.String middleName, java.lang.String lastName,
		java.lang.String nickName, java.lang.String prefixId,
		java.lang.String suffixId, boolean male, int birthdayMonth,
		int birthdayDay, int birthdayYear, java.lang.String jobTitle,
		java.lang.String organizationId, java.lang.String locationId,
		boolean sendEmail)
		throws com.liferay.portal.PortalException, 
			com.liferay.portal.SystemException, java.rmi.RemoteException;

Add a new user inserting in its profile the provided information

	public static com.liferay.portal.model.User updateUser(
		java.lang.String userId, java.lang.String password,
		java.lang.String emailAddress, java.lang.String languageId,
		java.lang.String timeZoneId, java.lang.String greeting,
		java.lang.String resolution, java.lang.String comments,
		java.lang.String firstName, java.lang.String middleName,
		java.lang.String lastName, java.lang.String nickName,
		java.lang.String prefixId, java.lang.String suffixId, boolean male,
		int birthdayMonth, int birthdayDay, int birthdayYear,
		java.lang.String smsSn, java.lang.String aimSn, java.lang.String icqSn,
		java.lang.String jabberSn, java.lang.String msnSn,
		java.lang.String skypeSn, java.lang.String ymSn,
		java.lang.String jobTitle, java.lang.String organizationId,
		java.lang.String locationId)
		throws com.liferay.portal.PortalException, 
			com.liferay.portal.SystemException, java.rmi.RemoteException;

Update a user's profile with the provided information.

	public static void addGroupUsers(java.lang.String groupId,
		java.lang.String[] userIds)
		throws com.liferay.portal.PortalException, 
			com.liferay.portal.SystemException, java.rmi.RemoteException;

Add a set of users to a give community (aka Group) identified by the groupId.

	public static void addRoleUsers(java.lang.String roleId,
		java.lang.String[] userIds)
		throws com.liferay.portal.PortalException, 
			com.liferay.portal.SystemException, java.rmi.RemoteException;

Add a set of users to a give Role identified by the roleId.

For more information check the Portal Javadocs.