See linked to knowledgebase item below for more information on certificates. The same rules that apply to adding a joint owner to a new certificate apply to a savings/checking account.
Currently in online banking we only display names of existing joint owner relationships, not relationships that are only co-borrowers or beneficiaries. The displayed people must be configured as a joint owner (J-type) in the SECNAMES file. If the member wants someone else added, they can contact your credit union, and your staff member can assist them in CU*BASE.