IEX Group, the upstart firm that gained fame by positioning itself as an antidote to high-frequency trading, has approached investors about raising money to help it become a full-fledged exchange, according to people with knowledge of the matter.

The company has yet to complete the amount of new capital to be raised, but it is expected to value IEX at between $200 million and $300 million, the people said. It has met with venture-capital funds, private-equity groups and asset managers about their potentially investing in IEX.