Easy is a relative term. To participate in a primary they pay for and retain their preferred status, unaffiliateds must affiliate with a party — which can be done up to and including on election day — and then change their affiliation back after the election. While it sounds easy, asking someone who views themselves as independent to declare to be something they’re not is a barrier to participation.
We know from general-election turnout in Colorado that unaffiliated voters are active and interested. The best — and most fair — way to increase participation iis to treat unaffiliateds the same as Republicans and Democrats, which is to mail them a ballot.