The leading members attended the meeting.
There are three main reasons.
Standard kanji spelling for this pre-noun adjectival.
Alternate kanji spelling using 立; less common but still encountered.
Rarely used kanji form; learners are unlikely to encounter it.
Derived from the adjective 主 (おも, 'main') combined with the past tense form of the verb 立つ (たつ, 'to stand'), literally 'that which has stood out as main'. The form 主だった functions as a fixed pre-noun adjectival.