The SSID can be 0 to 32 octets (although zero is special, so 1-32 is
usual). Where an octet is an (unsiged) 8-bit value. The 802.11 standard
does not impose any restrictions on the values. A @ would certainly be legal,
as are spaces and all the other usual punctuation. (I've seen a few SSIDs
that are simply one or more spaces.)
However, some cards and interfaces probably limit what you can enter for SSIDs.
....I haven't actually checked to see if an @ is legal in the SSID. Maybe I should and see if I can put in my email address...