Hi my goal is to have as easy as possible to manage a number of jomsocial sites, where users of site 1 can also be user of site 2. i see that multi sites can solve this, but there are some things that are not quite clear to me.

with multi sites, a common database is shared between sites. thus users of site 1 can be users of site 2 depending on access rights. my problem is with jomsocial. will the groups in site 1 be the same groups in site 2? or will the groups in site 1 be also visible in site 2?

thanks

Concerning JomSocial, it is effectivelly possible to share the users between the websites.
In this case, this will share all the JomSocial content except the configuration.

So in terms of JomSocial group they will be shared.
So the group in Site 1 will also the group in Site 2

just to clarify ... i thought there is like a way where two sites maintain different databases. like to separately installed joomla sites, so different jomsocial groups, but they somehow share user names and passwords or at least sync with some basic user fields like the username and password only.

is this possible?

You can have separated DB for the websites but in this case you not not share the JomSocial user.
All you could do is share the Joomla user but not the JomSocial info.

So you could only share the joomla username and password.

This should be possible.