dynamic user groups

Is it possible to dynamically allocate a new user group to an existing session on Solaris 5.8
I'd like to be able to allow certain users to access a set of scripts for the life of session (preferably there own session not a specific login created for the purpose) by dynamically giving the session the required group at any time during the session lifespan.

I'm aware of suexec and sudo but they don't necessarily provide what I'm looking for

Is this possible ?