You can bulk upload contacts from Excel into a Textburst group by clicking into 'Groups' > 'Add New' > 'Bulk Upload'.
This upload can be up to 16MB (16240kB).
You can upload up to 4 columns of data. If you choose to upload the maximum number of columns then Textburst will take up to 100,000 mobile numbers in one bulk upload.
If you're just uploading mobile numbers then you can fill an entire spreadsheet which will allow for 1,048,575 numbers.
* Excel files allow slightly more data to be uploaded than CSV files. This information is based on CSV file format.