You could achieve this using following code:
import com.webMethods.portal.bizPolicy.impl.ContextFactory;
import com.webMethods.portal.bizPolicy.IContext;
import com.webMethods.portal.bizPolicy.biz.IBizPolicyManager;
import com.webMethods.portal.bizPolicy.biz.IBizPolicyNames;
import com.webMethods.portal.bizPolicy.biz.dir.IDirServiceBizPolicy;
import com.webMethods.portal.bizPolicy.biz.dir.IDirSystemBizPolicy;
import com.webMethods.portal.service.dir.IDirPrincipal;
import com.webMethods.portal.service.dir.IDirSystem;
import com.webMethods.portal.system.PortalSystem;
IContext context = ContextFactory.acquireContext(true);
IBizPolicyManager bpm = (IBizPolicyManager) PortalSystem.getPortalSystem().getBizPolicyProvider();
IDirServiceBizPolicy dirServicePolicy = (IDirServiceBizPolicy) bpm.getBizPolicy(IBizPolicyNames.DIRECTORY_SERVICE);
IDirSystemBizPolicy dirSystemPolicy = (IDirSystemBizPolicy) bpm.getBizPolicy(IBizPolicyNames.DIRECTORY_SYSTEM);
IDirPrincipal user = dirSystemPolicy.lookupPrincipalByID(context, “user UID”, IDirSystem.TYPE_USER);
IDirPrincipal group = dirSystemPolicy.lookupPrincipalByID(context, “group UID”, IDirSystem.TYPE_GROUP);
IDirPrincipal role = dirSystemPolicy.lookupPrincipalByID(context, “role name”, IDirSystem.TYPE_ROLE);
dirServicePolicy.addPrincipalToRole(context, user.getDirectoryURI(), role.getDirectoryURI());;
dirServicePolicy.addPrincipalToGroup(context, user.getDirectoryURI(), group.getDirectoryURI());;
#webMethods-BPMS#MWS-CAF-Task-Engine#webMethods