The agenda is the road map for any meeting. Each agenda item should have a suggested timing, and in theory, the number of items on the agenda will be calculated so the meeting will end within a reasonable time. The participants will be ready to discuss the items they know are going to be decided at the meeting, based on the agenda. The meeting material circulated before the meeting, was about the agenda items. If your group allows agenda additions at the time of the meeting, then all this planning goes right out the window.
Resist adding items to the agenda at the time of the meeting. Participants have not had time to prepare and better decisions are made when people have time to carefully consider the advantages and disadvantages of any decision. An item could be added to the agenda if it is an emergency, something that is both urgent and important. The group could have a policy on the characteristics of last-minute agenda additions. The participants could choose to vote on the addition, and it can only be added if the participants agree.