When I've selected a grouped transaction and several other ungrouped transactions, I'm surprised not to see an "Add to Group" button anywhere. I finally found the way to add transactions to a group without ungrouping and re-grouping ("Edit Transaction Group" within the "Edit Transaction" panel for the group, then search by payee or other criteria ...), but this was highly unintuitive for me.

This action should probably be disabled if multiple groups are selected, the way the "Group Transactions" action is (senselessly) disabled when any of the selected transactions is "recurring".