Members sign in and invitations
Administrators of a team or organization have the responsibility of defining how a member can sign in and who can invite him.
In the General tab accessible through the gear icon at the top of the left-hand navigation menu, verify settings for members sign in :
Allow members to sign in let users sign in without being invited
You should add an email domain restriction to be sure that only members with these emails domains will be able to sign in without being invited
You can also define settings for members invitations :
You can allow members to invite other members
You can also add a constraint to be sure members can invite other members only if their email domain follow the same rule as for sign in.
Nb : these invitations settings do not apply to administrators who can always invite members and can fulfill or not domain rules.
Administrators of a team or organization can allow or prevent guests invitations by other members.
More about limitations of guests users here.