Limit users per group

https://gitlab.com/gitlab-com/infrastructure/issues/1048 shows that users are now creating groups and mass-inviting people to it, for the purpose of spam. GitHub has had similar cases in the past IIRC.

Throttling wouldn't really help as clever spammers can just add delays between adding users. On the other hand, putting an upper limit on the number of members per group could help. This limit would be defined per group, and can be increased by an admin (similar to repository size limits). A default sane limit could be something like 500 (e.g. as a global setting).