You would have noticed that you couldn't add members to some groups. There is no problem with your account and you don't need to worry about it.
The admin of the group can restrict group members to add new users. Please get in touch with the admin of your groups where you are unable to add members.
Note: Group admin settings are currently available only on the latest android version. Please upgrade to the latest version to entail the best of this feature. We'll slowly roll it out for iOS and Windows users as well.