Complex sentence: On 28 April the organisers informed that 1/3 of the signatories had confirmed that they were interested in joining the social network and told about their plans of making groups for participants on the site and visiting cities with at least several dozens of participants.
Split into simpler sentences: On 28 April the organisers informed that 1/3 of the signatories had confirmed the interest in joining the social network. They also told about plans of making groups for participants on the site and visiting cities with at least several dozens of participants.