After the death of the company's owner, Grossen and a colleague were left in charge of the firm.

[1] He joined the Green Liberal Party, which was founded in 2007 and later opened a cantonal affiliate in Bern.

In 2017, the party president, Martin Bäumle announced his intent to leave the post.

[3] In the 2019 election, Grossen was re-elected and the Green Liberals increased the vote share to 7.8%, taking 16 seats.

As part of his party's climate plan, Grossen supported ending subsidies to cattle farmers to reduce meat consumption and increase payments to plant-based products.