If your frind is on AT&T, Boost, Cellular One, Nextel, Sprint, T-Mobile, or Verizon Wireless ask them to text ‘YES’ to 69937 to accept your invitation.

(If they are on any other carrire ask them to text ‘YES’ to 9543046522))

-OR-

If your friend is already a Myxer user (hopefully they are), ask them to go to the Account -> Friends Settings page and accept your invitation from there.