You must be a team owner to add members to the team.
A team can hold up to 5000 people.
If you're a team owner, go to the team name in the teams list and click the 3 dots More options
Click Add member
Start typing a name to add to your team. You can also add people outside your organization as guests by typing their email addresses.
When you're done adding members, click Add
Click Close. People you add to a team will receive an email letting them know they are now a member of your team and the team will show up in their teams list.