trigger sendMail on FeedItem (after insert) {
    List<id> grpIds = new List<id>();
    List<Id> userIds = new List<Id>();

    for(Feeditem f : Trigger.new){
        if(String.valueOf(f.ParentId).startsWithIgnoreCase('0F9')){
            grpIds.add(f.ParentId);
        }
    }
    List<CollaborationGroupMember> myGroupMems = [SELECT Id, MemberId FROM CollaborationGroupMember WHERE CollaborationGroupid =: grpIds];

    for(CollaborationGroupMember cgm : myGroupMems){
        userIds.add(cgm.MemberId);
    }

    List<User> finalUserList = [SELECT Id, Email FROM User WHERE Id =: UserIds];
    List<Messaging.SingleEmailMessage> mails = new List<Messaging.SingleEmailMessage>();

    for(User u : finalUserList){
        Messaging.SingleEmailMessage singleMail = new Messaging.SingleEmailMessage();
        singleMail.setToAddresses(new String[] {u.email});
        singleMail.setReplyTo('myCompanyAdmin@example.com');
        singleMail.setSenderDisplayName('Company Name');
        singleMail.setSubject('Someone posted something');
        singleMail.setHtmlBody('Sample body');

        mails.add(singleMail);
    }
    Messaging.sendEmail(mails);
}