Using custom background when on someone else's meeting

[inctrl]

An organization I belong to uses MS Teams for meetings.  I can join their meetings when I get an invitation, by clicking on the link they sent, but there's no option, while I'm in their meeting, for me to change my background.  I also have a free account with MS Teams, which says it allows for a custom background, but I can't find a way to join the other meeting while signed in to my account.

Is there a way to activate the custom background while I'm in a meeting that someone else started, either while I'm signed in to my free account or not?

[#2]

In theory, if you are using the Teams Desktop client to attend a meeting, you should be able to change your backgroud.

[#3]

I have the same problem, if I enter an externally organized meeting I cannot change the background nor apply the effects, in both cases I don't see anything in the list.
For display effects, I have checked and the processor supports them and I have a geforce, I have the updated version of teams desktop

[#4]

Me too.  That's why I came here to try to find an answer.

[#5]

@inctrl 

I also have this problem. I have the Microsoft Teams app installed on my personal laptop, but when an external company email address sends me an invitation, like for an interview, none of my loaded backgrounds populate. In fact the only option is real life or blurred. Please help!