How do I merge with another family member's tree

Since you are a 'pro' member it is significantly easier. The way i prefer to do that is:
1. Open up a profile within your tree.
2. In the URL bar select the number that starts with 6000... that is the id of the profile. And copy that number.
3. Search for the name of that profile and find the name inside your family member's tree and open that other profile.
4. You will see a button that says 'Actions' or something similar, click on it and from the menu select 'Merge profile'.
5. Paste the number in the field, and select more, after the name of the person appears select that.
6. Click merge, and then confirm.

Alternatively you can:
Open one family tree, find a common profile, you will need to select 'Move profile' (but how you do that depends on the interface you are using
Open another tree you will see that particular profile floating on your right
Find the profile that it (the floating one) is a duplicate of, and move it (the floating one) over its copy in this tree.
Answer that they are the same, confirm.

Alternatively -- if the person who created the other tree / other version is a family member you know -- ask them what e-mail they are using for geni, enter it in the e-mail field for the version of the profile on your tree -- and they will not even have to confirm the two are the same, they will simply be merged.

Private User Wow, that is dangerous. This means that i should create an address for Geni only and make it private, since otherwise i allow anybody to forcefully add themselves to my family group by just entering my e-mail into a single field.

Volodya Mozhenkov - If anyone enters your e-mail in a profile, then your profile would be merged with the profile they entered the e-mail in -- and you would contact Customer Service and have the situation corrected - and the Geni-user who did that would probably be kicked out.
If that scares you, you probably do not want to work on Geni, since -- there is nothing programmed in Geni that prevents any Geni-user from merging themselves or a profile closely related to them with any Public Profile on the World Tree - regardless of whether names or any info matches.
Just as with putting your e-mail in another Profile not of you, it could be reversed, and anyone purposely doing any such thing would probably see their account closed - but it stands as a possibility.

That is *not* how data security works.

And it is not about silly arguments akin to "you should not be here". It would take about a couple of hours to weaponise this bug. And trust me a script that does this would be nigh impossible to stop, since it takes much more time to fix the damage to a family tree than to create a new account and then start a script with that new account as an input.

I suppose i will create a separate e-mail to use only for geni to temporarily mitigate such problem, but i doubt that i'll be able to convince all my relatives to do the same.

Volodya Mozhenkov - Why is it you see
a Geni-user being able to enter the e-mail you use on Geni in another profile - which would result in that profile being merged with the one for you, and you being the only
one who could control that merged profile [they would have no more ability to enter info or etc on that merged profile than they do now on your own profile]

and a Geni-user being able to merge the profile of themselves or any of their unclaimed family members with any Public Profile in your Family

as not equally dangerous? Why is it you only see the first one as being dangerous? (Or see it as being way more dangerous?)

[PS - since you are a Pro, you could cut the connection(s) to the bogus family, before or rather than contacting Customer Service to have it corrected ]

Volodya -
If, for example, someone merges a parent or sibling of theirs with a parent or grandparent of yours, they will become a member of your Max Family
why are you way less worried about this than about the possibility they may put your e-mail in a profile of one of their relatives, and thus become a member of your Max Family tho lose control of that profile because it will become you, and you will be the only one that can control it?

Right now i am a little busy, but a little later i will try to write a weaponising script that destroys somebody's tree by abusing such a bug and send this script to support of Geni.

To answer your question as to the difference, the difference is that you can make yourself a much closer relative by doing so, thus entering into a person's "immediate family", which does give you more control.

Volodya Mozhenkov - One example - If someone merges a Great-grandparent of theirs with a Great-Grandparent (or even 3rd great with 3rd great) of yours, then you and they will be in each other's Max Family - so then they can merge any profile they want with any unclaimed Profile in your immediate family (if 3 great merged - only down to you, your siblings, and profiles managed by same) - even if Private - or attach themselves as your sibling, etc. -- so possibly it might take two steps instead of the one that can be achieved with entering a e-mail address - so do not see the bogus entry of your e-mail in another profile as more likely to be a danger.

Moreover - since you as a Pro can cut the bogus connection(s), whether from a merge via e-mail entry or thru a bogus merge to an unclaimed profile in your Family - cannot imagine why you are in such a panic.

Geni is a designed as a Collaborative site - the down-side of such is that they rely on folks acting responsibly in order to function properly.
Geni has ways for corrections to be made when that does not happen, and to remove folks who are purposely malicious.
Personally, I think that is as good as can be expected.

I do not understand what you do not understand about what has been said in the last message. I have nothing to add at this moment, except that i will investigate the other bug you have talked about and see if it actually can add a person to their immediate family.

Volodya - we are not talking about bugs here.
We are talking about errors that can be introduced if someone chooses to purposely misuse the system - by either purposely entering false information (as is the case if they enter an e-mail in an inappropriate profile for that e-mail) or purposely merging profiles that are not actually of the same person.

And - as I have mentioned - Geni has ways for corrections to be made when that happens, and to remove folks who are purposely malicious.
However, it is certainly one reason that most folks recommend Geni not be your sole repository of info, and many also have their database/genealogy on a separate program on their own computer.