public DistributedMember findMember(final String memberName) {
    return ManagementUtils.getDistributedMemberByNameOrId(memberName, (InternalCache) getCache());
  }
this will return the member found or null if no member with that name

@param memberName the member name of the member to find
@return the member found or null if no member with that name exists
geode-gfsh/src/main/java/org/apache/geode/management/cli/GfshCommand.java