If you are a BT Broadband customer you are automatically opted in.
Hi @Alisa,
Welcome to the Community and thank you for posting.
What @licquorice has posted is correct. Since March 2009 all new BT Broadband customers are automatically opted into BT Wi-fi.
You can find out more here - How do I check that I've joined the BT Wi-fi community?
Thanks,
Paddy