These questions are dealt with here:
The Shulchan Aruch (OC 551:9) writes that one mustn’t eat meat or drink wine during the nine days. The Rema (OC 551:10) adds that if one has a seudas mitzva during this time then one may partake of wine and meat as such celebrations are incomplete without them. This includes Shabbos meals, a bris seuda, a pidyon haben, or a siyum.
The Aruch Hashulchan (551:28) strongly disapproved of postponing one’s siyum to coincide with the nine days in order to eat meat, though writes that if one was learning something (extra) and planning on making the siyum then, that is commendable (See Mishna Berura 551:28). The Lubavitcher Rebbe encouraged people to specifically make a siyum during this time, saying it would add light to this dark period (though his minhag was not to eat meat then). Other poskim write that one should avoid making a siyum afer the 6th of Av (See Moadei Yeshurun p132).
The Mishna Berura (551:75) writes that anyone who would normally be invited to one’s siyum may participate and eat meat, too, though one shouldn’t invite extra people.
While some poskim hold that only one who was present for the learning and siyum may partake of the meal, the consensus of poskim (Minchas Yitzchak 9:45; Rivevos Ephraim 3:343; Teshuvos Vehanhagos 1:300) is that one who came late mat still join in and eat meat (See Mishna Berura 470:10).