if (this.sampleArr.length === 0) {
      this.afs.collection('users',ref => ref.where('displayName','==',firstLetter))
      .snapshotChanges().subscribe(data => {
        data.forEach(childData => {
          const uid = childData.payload.doc.data();
          const isMember = ((this.groupInfo.superUser.includes(uid.uid))
                          || (this.groupInfo.administrators.includes(uid.uid))
                          || (this.groupInfo.members.includes(uid.uid)));
                      console.log('ismember',isMember);
          if (!isMember) {
            this.sampleArr.push(childData.payload.doc.data());
          }
          console.log('this.sampleArr',this.sampleArr);
        });
      });
    }