
der Gesellschafter
partner
A person or company that owns shares in a company, such as a GmbH or KG, and is therefore a co-owner.
Die beiden Gesellschafter haben beschlossen, die Firma zu verkaufen.
The two partners decided to sell the company.
partner / companion
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partner
A person or company that owns shares in a company, such as a GmbH or KG, and is therefore a co-owner.
Die beiden Gesellschafter haben beschlossen, die Firma zu verkaufen.
The two partners decided to sell the company.

companion
A person who keeps someone company, often as a paid companion on trips or at social occasions.
Die reiche Witwe suchte einen gebildeten Gesellschafter für ihre Weltreise.
The wealthy widow was looking for an educated companion for her trip around the world.
The word is derived from 'Gesellschaft' (company/society). This word can refer both to a company in a business sense and to social togetherness, which explains the two meanings of 'Gesellschafter' (partner/companion): a partner in a company or a social companion.
Er ist einer von drei Gesellschaftern der neuen IT-Firma.
He is one of three partners in the new IT company.
You can recognize the meaning from the context. If it is about companies, shares, or contracts, it means the co-owner. If it is about travel, accompaniment, or social occasions, it means the companion.
Die Gesellschafter treffen sich einmal im Jahr zur Versammlung.
The partners meet once a year for the assembly.
In a business context, there are fixed terms like 'geschäftsführender Gesellschafter' (managing partner, actively runs the company) or 'stiller Gesellschafter' (silent partner, only financially involved). The legal form is also often mentioned, e.g. 'Gesellschafter einer GmbH' (partner/shareholder of a GmbH).
Als stiller Gesellschafter hat er kein Mitspracherecht im Tagesgeschäft.
As a silent partner, he has no say in day-to-day business.
The meaning 'Begleitperson' (companion) is rather rare today and often sounds old-fashioned or very formal. You find it more often in historical novels or films than in modern everyday language.
In dem Roman sucht ein älterer Herr einen Gesellschafter für seine Spaziergänge.
In the novel, an elderly gentleman is looking for a companion for his walks.
companion / accompanist
Begleiter generally refers to a person who travels with someone or goes with them to an occasion. In this context, Gesellschafter can also mean such a person, but it often refers to someone who is paid to provide company. Without this special aspect, Begleiter is the better fit.

Paul fährt als mein bezahlter Gesellschafter mit mir zu der Feier.
Paul is coming with me to the party as my paid companion.
Paul fährt als mein Begleiter mit mir zu der Feier.
Paul is coming with me to the party as my companion.