If you have more than a few clients to invite, use Client records → Invite several.
What you fill in. Two things per client, both required: name and email address. Nothing more is needed; the client fills in the rest themselves in the app. A line without a name, with an invalid address or with an address that appears twice turns red and isn’t included.
Pasting from a list. With Paste from a list you put a whole list into the form in one go, for example two columns from Excel or a list from an email. One name and one email address per line, separated by a semicolon, comma or tab.
Sending. Each client gets their own invitation, the same as with Add new client. You see per line whether it worked. Anyone who already has an account doesn’t get a new email but is linked. If the mailer briefly calls a halt partway through, wait a few minutes and choose Continue where I left off; whatever has already been sent doesn’t go twice.
Offering a subscription straight away. Tick Also offer a subscription straight away and choose a subscription. Everyone who has just been invited gets it as a quote. See Offering a subscription.
A file? You import a CSV file under Member management. See Importing members.