The grammar rules say that 'either' or 'neither' are used to agree with a negative statement and 'also' and 'too' are used to agree with positive statements. But in informal language people often use 'also' and 'too' for negative agreement as well..
so grammatically the agreement to 'I can't have fun' would be a) I can't have fun either or b) Neither can I.
But, 'I, too, can't', 'I, also, can't' or 'me neither' are often used.