If they are using the app - no - they will need to switch tenant in order to access your Team.
However, they could open teams in their web browser, switch to you tenant in the browser, then use that browser session to access your Team, and leave their Teams app for accessing their own tenant Teams.
Yes, they must be a guest and must tenant switch to access your Team. There are ways to be in two tenants at once as Rob pointed out. A browser tab open switched to the tenant, or utilizing a tool like Franz if you want a more desktop client like experience.