See:
For “participation share”, compare item 2 on the Greek language page of this website:
https://www.golfinopoulos.com/el/6130/katargisi-kathe-perior...
With the corresponding paragraph of the English language page of the same website, here:
https://www.golfinopoulos.com/6135/abolition-of-all-restrict...

and

https://el.dictionarist.com/ελληνικά-αγγλικά/μερίδα συμμετοχ...

19 Ιουλ 2010 — ... μερίδια εκάστου εταίρου αποτελούσι την μερίδα συμμετοχής αυτού. ... one company parts of each partner form the participation share thereof.
https://www.translatum.gr/forum/index.php?topic=78026.0

For the remainder of the text, see:
The Oxford Greek/English Dictionary gives one of the meanings of ‘περιέχομαι’ to be ‘come to’,
So:
5 This means that if one share comes to more than one person, the owners should be considered co-owners of the ab indivis share and the exercise of rights ...
https://www.academia.edu/38574364/The_disruption_of_the_prin...


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Note added at 19 hrs (2021-02-28 09:57:50 GMT)
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However, I also agree with Nadia-Anastasia Fahmi in that ‘μερίδα συμμετοχής’ could also be translated (in this instance) simply as ‘share’, because the term ‘participation share’ is normally used in English to mean a person’s total share participation (the total quantity of shares that a person has) in a company or business, and, if you have a share in a company, you generally participate in that company whether you do so actively or not.
See:
https://www.proz.com/kudoz/greek-to-english/law-general/1694...
If you were to regard that argument as being plausible (as I do) an alternative (and possibly better) translation of your text would be:
‘if one share comes to more than one person’, which is covered in the https://www.academia.edu/38574364/The_disruption_of_the_prin... Internet link above.