Jean Urquhart, who sits in Holyrood as an independent MSP for the Highlands, backed the pro-independence alliance when it launched in August.
Rise aims to provide a socialist alternative to the SNP and Labour.
It hopes to win seats in Holyrood through the regional list system.
The most recent meeting of Rise saw a proposal for a Highlands and Islands manifesto to "reflect the unique needs of the region".
Ms Urquhart said she was "delighted, as a member of this new and dynamic movement" to be involved in considering the manifesto.
She said: "The strength of Rise is the grassroots involvement in every area, and I welcome the interest that it is generating already."
Rise - which stands for respect, independence, socialism and environmentalism - was born from the wider "Yes" movement in the wake of the independence referendum campaign.
It includes the Scottish Socialist Party, the International Socialist Group, and other campaign groups.
Ms Urquhart was one of three SNP MSPs who left the party in the wake of it voting to end its opposition to an independent Scotland joining Nato.
The other two - John Wilson and John Finnie - have since joined the Scottish Greens.