What possibilities does a private person has to merge his capital (mostly stocks) with 2-3 more different private persons? Apart from having lesser transaction costs, which advantages or disadvantages could we have?
You can form an investment club with other people.
Some of the benefits include:
Inexperienced members and poor results
Investment clubs tend not to be regulated by the SEC but in some cases they are
Disagreement on how to trade/invest
Difficulty withdrawing money if you want to leave the club
Loss of autonomy
In the U.S., the LLC or partnership for investing must be a non-issuer company or else it would be subject to regulation as an investment company. But personnel running the LLC or partnership, or party to it, might be subject to broker/dealer regulation or to investment advisor regulation.
Now if the non-issuer company is claiming investing as its core operation then I suppose that it is not subject to capital-gain tax but to one of two methods of corporate income tax. Also, expenses could be charged against earnings.