A team can hold up to 5000 people.
1. If you're a team owner, go to the team name in the TEAMS list
Click More Options [...] > Add member
2. Start by typing a name, distribution list, security group, or a Microsoft 365 group. You can also add people outside your org as guests by typing their email addresses.
3. When you're done adding members, select Add. You can make someone a team owner by selecting the down arrow next to Member. A team can have multiple owners. For more about roles and permissions, see Team owner and member capabilities in Teams.
4. Select Close. People that you add to a team will receive an email letting them know they are now a member and the team will appear in their TEAMS list.