I have many many XOOPS sites.
Now what i want to do is enable the members to register in one of the site and they are able to login in all the XOOPS site i run.

How do i do this?
Export which database ?

Do you want it to be like that automatically?

There are some other posts as to running several xoopses on one database, but else, I guess you'd need to copy the tables for users and permissions from one site to the others.

xoops_users is the base table for users. I have copied the table to give a reinstall a head start and it works.

There are a few threads around (active this morning) about various schemes for running multiple sites, so I won't rehash that. Have a search.

Something else that could be an easy hack is to make a symlink (Unix) of the _users table to the other databases. This is of course after the initial install and then renaming the _users table in the destination db.

This will get your users on all sites. Each site would still be totaly unique in content and you have to configure groups rights on each.

I don't like symlink MySQL tables as you have to watch out for a few things down the road but it does work.

Another way is to use MySQL Replicate and pick only the tables you need duped. But that is another story.

You could even hack the db class to query a _users table from a common db but that is yet another story. LDAP, etc etc.