Labour and the Liberal Democrats have rejected a Conservative call for a pro-Union alliance.
This is despite Alex Salmond’s threat to forge an independence “supermajority” with his new Alba Party.
Scotland in Union said the pro-UK electorate could unite to thwart Ms Sturgeon’s party in many key constituencies.
Its guide points to the “best chance” party to stop the SNP across the 73 first-past-the-post seats.
The group suggests voters back their preferred pro-UK party – the Tories, Labour, or Lib Dem – on the regional list.
Smaller parties such as George Galloway’s All For Unity and Reform UK hope to gain some of these 56 top-up seats.
But Scotland in Union warned supporting minor groups would only help the SNP.
Its guidance is based on a number of factors, including the results of 2016, 2017 and 2019 elections, polling results since then, and local information.
The voting suggestions do not constitute an endorsement of any candidate or party’s policies, Scotland in Union said.
Former Labour and Respect MP Mr Galloway is standing in the South Scotland region for his party.
He said: “I have no idea who Scotland in the Union are – they don’t cut much mustard on the streets of Scotland, or they’d be standing for election themselves.
“We are standing and winning in all eight regions of Scotland, and our plan is now common currency amongst Scottish voters.”
A Scottish Conservative spokesman said: “Only the Scottish Conservatives can stop an SNP majority and their plans to hold another divisive referendum in the next two years.
“If pro-UK voters unite around us on their party list vote ballot paper, then we can fully focus on our recovery and thwart nationalist plans for indyref 2.”
A Scottish Labour spokesman said: “Unlike the SNP and the Tories, working to deliver a real national recovery plan without an unnecessary referendum will be our only priority.
“Scotland deserves a better government and a better opposition – and the only way to get that is to cast both your votes for Scottish Labour.”
An SNP spokesperson: “This is nothing more than a desperate bid from a failing campaign that has no positive policies to offer to improve people’s lives.”
The Scotland in Union tactical voting guide can be visited at www.scotlandinunion.co.uk/tactical-voting-guide-2021