Hello Daniele, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
Loans to companies by directors, shareholders and [misguided - joke] friends are common features in companies setting up in business. It is a quick way of obtaining favourable cash flow. The basics of SEIS, a tax relief measure for individual investors, are set out here:
Having loans by individuals to a company in place does not impinge on the viability of SEIS.
There are a lot of complex rules relating to SEIS and many of them are quoted here;
It is pitifully easy to breach these these restrictions.
I do hope that you have found my reply of assistance.