If you have a large number of opt-in members that you want to add to your Moosend mailing list, the best course of action is to upload a .csv .txt .xls or .xlsx file which contains your member list. The size of your file cannot exceed 40 MB but that's about the only limit you'll run into during the import process.
You can find the various import options on the Import members page. Just follow the steps described below:
1. Click on the Mailing lists button, located on the left side of your Control Panel page. 
2. Click on the name of the mailing list to which you want to add more members. 
3. Click on the Import Members button. 
4. Select the import method which best suits your needs! 
Not sure which one to choose? You can learn all you need to know about each method and find the one that works for you!