It will depend. GroupWise has two different password approaches.
- User a distinct GW password, stored in the GW databases
- User an LDAP password, usually pointed against eDirectory
If the first case, I am not sure how you might do it, though I suppose a POP connect in Java testing for success would work.
If the second case, then you would just need to know the eDirectory LDAP server information, and connect to that. Get the server, whether it requires SSL/TLS or not, an account with sufficient rights to find users in eDirectory, and then decide what attribute you will use to find users. Probably uniqueID in eDirectory (uid via LDAP) but you will need to ask if it is populated in this system. Due to historical vagaries of eDirectory, early tools did not always set uniqueID on users. Thus it is possible not every object has it set. However, all modern tools set it, so it would only be a one time fix. CN is a bad choice as it can be multi valued. Of course, if email address are populated, you could require mail (Internet EMail Address in eDirectory).
Basically how you would do any standard LDAP integration. There are libraries like LDAPChai to make this easier as well.