One Flex Mobile account can cover a whole household, with a separate line for each person. Not everyone on the account has the same powers, and knowing the difference saves a lot of confusion.
The account holders
The person who set the account up is an account holder, and they can invite others to become account holders too.
Everyone with this role has the same reach over the whole account: buy new lines, change plans, pay, manage the card, see the billing, and manage every line on the account, including lines that aren't theirs. They can also invite further account holders.
Think of it as a second parent. Give it to someone you'd be comfortable having change the plans and see everything you're billed for.
Everyone else on the account
A person can have a line without being an account holder. They have a phone number and service, and they can see what's going on with their own line, but they can't buy new services, change plans, or touch the payment details.
That's the usual setup for children on a family account. They get a phone that works, and nobody can accidentally order something.
The contact person
One account holder is marked as the primary contact person for the account. That's who Flex Mobile reaches out to about the account as a whole. Any account holder can take over that role, and there's only ever one at a time.
A practical example
A parent sets up the account and invites the other parent as a second account holder so both can manage everything. Two children each get a line but no administrator rights. Four lines, four phone numbers, one bill, and only the two adults can change anything.