The third option is to use the CSV files to import all of your data and to automatically setup and invite all of your users.
There are variations of this approach that are also possible, and which provide a semi-automated solution to pre-load as much data as possible.
A fully automated solution requires that the three CSV worksheets be completed with ALL Groups and ALL Staff and ALL Client Users, as well as ALL of the group memberships correctly populated in the staff and client CSV files.
Only under this approach can you actually also safely send all invitations as part of the CSV import process for clients.
If your data is incomplete (i.e. not all client details are complete) or relationship data required to create the client groups does not exist, an alternative would be to import all client users into a dummy group that allow the data to be loaded but the invitations not sent. Dummy values can be used to populate the mobile phone numbers and email addresses if required. Remember the email must be a unique value.
Under this approach, the creation of the correct client group and memberships are performed manually ahead of manually checking to correct the client user record and triggering the invitation process.
Remember to load groups, then staff, and only once staff members in the client groups have accepted their invitations can you load your clients.