The potential to add another person to your individual account depends upon their intended ability or role. The 4 most familiar roles are the following:
Owner - account holders may not include another person to an existing account, if the result of adding that individual would serve to change account ownership. If, for example, you are seeking to convert your individual account into a joint account, you would be required to initiate a joint account application, engage in the agreements correlated with that account and request a transfer of assets thereafter. See Converting From a Single to Joint Account.
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Beneficiary - the addition of a beneficiary or nominee is supported for a limited set of account types (e.g. a Transfer of Death for U.S. individuals and IRA accounts).

See How can I designate a beneficiary on my account?
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User - account holders might add a non-employee individual as a user and allow them certain account access rights (e.g., reports, trading). Note that adding a user to assist with operating and/ or keeping track of the account implies no charge in ownership.

See Adding (non-employee) users to an individual account for information on adding a non-employee user.
Trusted Contact - an individual who MEXEM would be accredited to contact and disclose information regarding their account to address potential financial exploitation, to verify the specifics of the client's latest contact information, health status, or the indentity of any legal guardian, trustee, executor or holder of a power of attorney. This individual is not provided with entry to the account nor are they authorized to accept any transactions for the account.

See What is a Trusted Contact?