This problem has been faced by many folks..Many times when you click on Assign Access for webforms/Dimension Members. There can be many reasons for this.
1. Check all the services are running fine.. Hyperion services create problems most of the times.. So, just restart services in order, it works in most of the cases.
2. Some issue with RDBMS, so just catch your DBA to get work done.
3. Some mis-sync between LDAP - SS . So, Try going to http://localhost:58080/interop/framework/configLogin
Configuration > Sync OpenLdap
4. Just check that users have access to planning app and users are planners and not Admins, as admins won't appear in the list (I know this reason sounds funny but one of my friend made this mistake and we tried a lot of things).