1. The Gemara states that twenty four thousand students of Rabbi Akiva died during the period of Sefira, which is the time between Pesach and Shavuos. Since Talmudic times, the period of Sefira is treated as a time of mourning and the conducting of weddings is prohibited during this season.
2. There are various customs regarding which days of Sefira are to be kept as a period of mourning and are therefore not times for a wedding to take place. Many refrain from getting married from Pesach until Lag B’Omer, while others refrain from marrying from Rosh Chodesh Iyar until three days before Shavuos (there are other customs, see Rama and Mishnah Berurah).
3. Many times a chosson and kallah will keep the first half of Sefira and wish to make their wedding after Lag B’Omer. The poskim discuss whether someone who is keeping the second half of Sefira can come and dance at a wedding taking place during their Sefira.
4. Harav Moshe Feinstein zt”l (Igros Moshe 159) writes that they are permitted to attend and even dance at the wedding. In addition, a rabbi can serve as the mesader kiddushin (orchestrate the wedding) even though he is still in the midst of his Sefira. He explains that the prohibition of getting married during Sefira belongs only to the chosson and kallah and when they are permitted to marry, others are permitted to attend. Harav Shlomo Zalman Auerbach zt”l agrees with the ruling of Harav Moshe and he allows for people to attend weddings during Sefira. He himself would also travel to weddings after Lag B’Omer, even though he was still keeping Sefira. He would, however, avoid being mesader kiddushin during that time (Shalmei Simcha page 84). Harav Yosef Shalom Elyashiv zt”l (Ashrei Haish vol. 3 65:30) and Harav Yaakov Kamanetzky zt”l (Emes L’Yaakov 493) rule leniently as well.
Harav Yitzchak Weiss zt”l, however, in his Sefer Minchas Yitzchak (4:84), disagrees and feels that one may not attend a wedding during their Sefira, even if the chosson and kallah are not keeping Sefira. It seems that the common custom is to rule leniently in accordance with the view of Harav Moshe Feinstein