You cannot easily. I am not even sure if there is any way to make this work. The only thing I can think of is to create a separate ip pool of 1 address for each user, and have a separate vpngroup config for each user:
vpngroup user1 address-pool pooluser1
vpngroup user1 dns-server....
vpngroup user1 .....
vpngroup user2
Each user would have to configure their client with their vpngroup user name, and its password.