You are free to join Animo whenever you want. You pay a fee of €12,50 for one academic year, for which you get a book discount, loads of formal and informal activities and the opportunity to become active next to your studies. So no reason to doubt!
You can find the application form here:
To finalise your membership, you are required to pay the membership fee of €12,50 to NL RABO 0152 2845 08 under the name of SV Animo, mention 'name + membership fee' in the description!