When I signed up to BT Mobile an 'agent' said I could add another sim to the account when that person was ready to switch over - that person being my wife, and her contract with O2 has come to an end.
Following some of the 'help' sections on the My BT, I'm not able to find any of the menu options or pages that should be accessible to add another sim to our account.
I'm sure it's likely I'll have to call tomorrow, but was wondering if there was a working way to do it online.
On MyBT, select products then Mobile. There should be an option to change your plan. We're already enjoying the Family Plan so I cannot test my suggestion.