The only way to join a team in Cisco Spark is if a moderator adds you to it. Sometimes you'll just get added to a team because you're part of a project or department. Or, if you know about a team because someone you work with is already a member, you can reach out to the moderators to get added. But, you can't just search for a project and find all available spaces and teams associated with it. If you're not already a member, they won't show up in your search results.
If there's a team you need to join, contact one of the team's moderators to add you to it.