I have 5 existing Joomla websites all on the same server and under the same domain name. All are fully functional with their own joomla installation and database.

Example:
mainsite.com; Joomla 1.5.22 and database1
mainsite.com/2; Joomla 1.6 and database2
mainsite.com/3 Joomla 1.5.22 and database3
mainsite.com/4; Joomla 1.6 and database4
mainsite.com/5 Joomla 1.5.22 and database5

Can I install JMS so that I can share components, users, etc. among them?

Thank you.

You can install JMS Multisites but you will not be able to share the users.
The sharing of an extension mean that you share the MySQL table.
That means that you need to have exactly the same user ID with the same info in all the website if you would like to hope delete all the users table in all the website and just keep one.
This is not possible.

In addition, you mentioned Joomla 1.5 and 1.6 in your list.
Their management is totally different and the management of the extension is also different.

So NO you can not share extension between existing website.

The sharing is something that you decide when you create the slave site.
The sharing does not work across Joomla version.

Can: Step 1) users be exported via another component or SQL query from existing sites (those that will become slave sites), and then imported to the primary site. Step 2) users deleted from slave sites. Step 3) JMS Multisites installed and existing and new slave sites set up to share users of main site?

1) You can always export the users with MySQL but it will not be easy to append them to an existing website as all the User ID should be modified and perhaps the tables.
I don't know tool that allows doing that and import a list of users into Joomla.

As you seems that you want sharing the user with an existing website, change the user ID is not good as you could have the user ID with different meaning.

In example, the users id 1 in site 1 correspond to person A
the same user id 1 in site 2 may correspond to a person B.

So if the user id 1 write an articles in the site 2, the author will be the person B.
If you import the user 1 from site 2 to add it in site 1 with id 2 then the article present in site 2 will not be assigned to the person A (that is totally wrong).

In summary, you can not import user and change the user ID or you create inconsistencies.

2) Yes you can only delete the users after you have import them with exactly the same user ID into the previous site.

3) Once you dropped the users and all the linked tables, you can effectivelly rebuild a rule that consists in the sharing of the users.

The critical part is to guarantee the consistency of the users ID with all the tables used by all the extensions of your websites.

Thanks,

The registration of users is an area of growing importance as social networks grow. Joomla should address this better in the core functionality so that extensions of various types can work across domains.

Okay... and..

in following situation.

master =db1 (J1.6 with components)
slave =db2 (J1.6 with no components)

db2 is copy of db1
only without components but with users.

so in db1 user ID 200 is mr. X
and in db2 user ID 200 is also mr. X

Is it in this situation possible to share users on existing sites?
Or is it better to create a new install of slave site.