Although question is not quite clear to me, because:

==case 1==

if you have multisite WP, then users are shared with same username/password/etc). Just you should set the roles individually per sub-site. But if there are many users and you want to automatize the process, you can use:

• https://wordpress.org/plugins/multisite-user-sync/
• https://wordpress.org/plugins/multisite-user-role-sync/
• or some similar alternatives

update: As I see, you only need to “add” (not re-create users, but just “add”) into the existing sub-domains. Go to http://your_site.com/wp-admin/network/sites.php, click each listed site individually> go to Users tab:

and insert username (or mail) into Existing user > choose Role and click add. (to automatize the work, if there are tens of users and subsites, you can use above plugins.

==case 2==

If you have separate installations of WP (and each WP installation is MS itself), then the possible way is like the previous answer suggests (sharing the user and usermeta tables), but note, there are cases, when in wp_options table there could be some information too for specific user, which wont be shared to second site in this case, and you will have to manually share… Not a quite easy trick, but you should live with that.