Toronto

That's not how tenant unions work. I'm not in Toronto, but collecting a list of these unions and having a look at them might be useful.

Often times there are ones for particularly vulnerable groups, like new immigrants or single mothers. Rather than tenants have to figure stuff out on their own.

I did a quick search and found the Federation of Metro Tenants' Associations. Becoming a member is more about supporting and donating over time, much like you would any organization you believe in whether as an individual or a business.

The services of these organizations are free for tenants, like the Toronto Renters Forum that the FMTA runs https://www.torontotenants.org/toronto_renters_forum

Standing in solidarity might mean sharing their stuff, supporting their point of view, or any number of other supportive, joint action.