You can download the list of clients in your database to a spreadsheet ready for upload to other software e.g. Mailchimp.
Go to Admin > Clients
Click the blue options buttons next to the search bar
Click "Export as CSV"
The columns included are:
First Name
Surname
DOB
Age
Mobile
Email
Address 1
Created
First visit
Last visit
ID
Tags
Email Marketing
SMS Marketing
If you want to update your mailing list only with those that were created within the last month for example, filter by the Created column.
To prevent staff from being able to do this, you can block their access to Admin.