how to get the checkbox values and storing into string array in android

You can add this method to your adapter:

public ArrayList<FriendInfo> getCheckedItems(){
    ArrayList<FriendInfo> result = new ArrayList<FriendInfo>();

    if(checkBoxState!=null){
       for (int i = 0; i < checkBoxState.length; i++) {
           if(checkBoxState[i]){
               result.add(getItem(i));
           }
       }
    }
    return result;
}

To call it: friendAdapter.getCheckedItems();

UPDATE :

So your code will be like that:

    public class GroupList extends ListActivity 
    {

        boolean[] checkBoxState;
        boolean isChecked;
        String check;
        ListView users;
        int position;
        private IAppManager imService = null;

        private FriendListAdapter friendAdapter;

        public String ownusername = new String();

        private class FriendListAdapter extends BaseAdapter 
        {   
            @SuppressWarnings("unused")

            class ViewHolder {
                TextView text;
                ImageView icon;
                CheckBox check1;



            }

            private LayoutInflater mInflater;
            private Bitmap mOnlineIcon;
            private Bitmap mOfflineIcon;        

            private FriendInfo[] friend = null;





            public FriendListAdapter(Context context) {
                super();            

                mInflater = LayoutInflater.from(context);

                mOnlineIcon = BitmapFactory.decodeResource(context.getResources(), R.drawable.greenstar);
                mOfflineIcon = BitmapFactory.decodeResource(context.getResources(), R.drawable.redstar);

            }

            public void setFriendList(FriendInfo[] friends)
            {
                this.friend = friends;

            }


            public int getCount() {     

                return friend.length;
            }


            public FriendInfo getItem(int position) {           

                return friend[position];
            }

            public long getItemId(int position) {

                return 0;
            }

            @SuppressWarnings("unused")
            public View getView(final int position, View convertView, ViewGroup parent) {

                final ViewHolder holder;


                if (convertView == null) 
                {
                    convertView = mInflater.inflate(R.layout.grouplist, null);


                    holder = new ViewHolder();

                    holder.text = (TextView) convertView.findViewById(R.id.text);
                    holder.icon = (ImageView) convertView.findViewById(R.id.icon);
                    holder.check1 = (CheckBox)convertView.findViewById(R.id.checkBox1);

                    convertView.setTag(holder);

                }           

                else {

                    holder = (ViewHolder) convertView.getTag();

                }


                holder.text.setText(friend[position].userName);
                holder.icon.setImageBitmap(friend[position].status == STATUS.ONLINE ? mOnlineIcon : mOfflineIcon);

                final ArrayList<String> checkedFriends = new ArrayList<String>();
                checkBoxState = new boolean[friend.length];
                holder.check1.setChecked(checkBoxState[position]);
                holder.check1.setOnCheckedChangeListener(new OnCheckedChangeListener(){

                    public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
                        checkBoxState[position]=isChecked;

                        if(isChecked){

                           check=friend[position].userName;

                        } 

                    }
                });


                return convertView;
            }
            public ArrayList<FriendInfo> getCheckedItems(){
                ArrayList<FriendInfo> result = new ArrayList<FriendInfo>();

                if(checkBoxState!=null){
                   for (int i = 0; i < checkBoxState.length; i++) {
                       if(checkBoxState[i]){
                           result.add(getItem(i));
                       }
                   }
                }
                return result;
            }
        }

protected void onCreate(Bundle savedInstanceState){       
            super.onCreate(savedInstanceState);

            if (android.os.Build.VERSION.SDK_INT > 9) {
                StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder()
                .permitAll().build();
                StrictMode.setThreadPolicy(policy);
            }
            setContentView(R.layout.group_list_screen);

            Button create=(Button)findViewById(R.id.create);


            friendAdapter = new FriendListAdapter(this); 
            /*
             * here you must load list items before calling 
             * "getCheckedItems()", I think you use "setFriendList(...)" for that 
             * so you need to do friendAdapter.setFriendList(YOUR_LIST). Otherwise you will have an empty array 
             * because there is no items, so no checked items too.
             */
            friendAdapter.getCheckedItems();

            create.setOnClickListener(new OnClickListener() {

                @SuppressWarnings({ "unused", "unchecked" })
                @Override
                public void onClick(View v) {
                    String groupname = getIntent().getStringExtra("nick");

                                try {
                                    /* 
                                     * you can also call "getCheckedItems()" here,
                                     * to send the checkedItems to "imService"
                                     */
                                    String result1 = imService.CreateGroup(groupname,imService.getUsername(),check);
                                } catch (UnsupportedEncodingException e) {

                                    e.printStackTrace();
                                }



                    Toast.makeText(getApplicationContext(), "Group Created Sucessfully",Toast.LENGTH_LONG).show();

                }
            });    

        }