After the latest Meccha Chameleon update, many players cannot find the old friends list or invite button. The current workaround is to open Find Server / Find Friend Lobby, search for your friend's lobby, then click your friend's server to join. If the lobby does not appear, make sure both players are updated, online on Steam, and still waiting in the lobby.
This is current player-reported guidance, not a guaranteed fix for every version, room state, Steam status, or host connection.