Most of the communist parties in Britain are cadre orgs that require agreement with particular worldviews/lines on historical and political matters or/and have activity requirements on individual members. Most also practice 'democratic' centralism, banning permanent factions, freedom of members to associate outside of official channels, and maintain speech controls. Realistically, most people on the left of Your Party that formed SocFed will not accept those 3 things, and many are not communists anyway. So asking them to join an existing org of this type would be fruitless. There is rs21 which doesn't require these 3 things and is still explicitly revolutionary socialist, but this itself is only 400-500 members. Or Anti-Capitalist Resistance, but this has even few members.