Can I invite a group of players at once?

Yes. Create contact groups in Account → Connections → Groups (e.g. "Weekend players" or "Mixed doubles"). When you click Invite players on your tournament, you can select one or more groups in addition to individual players; everyone in the selected groups is added to the invite list. If a group has optional filters (e.g. level or gender), only members matching those filters are included. Sending then creates one invitation per selected recipient.