'indicated their interests' is a correct and usable phrase in written English.
You can use it whenever you want to express that someone revealed or expressed what they are interested in or what they like. For example: The students in the class indicated their interests in sports, music, and art.
Since the Kursk submarine incident, the Russians have indicated their interest in talking to NATO about maritime safety.
Mr. Zhang said remarks by Iranian officials over the past few days indicated their interest in finding a diplomatic resolution.
When the Red Sox first indicated their interest in signing Millar, they were prepared to pay the Dragons $1.2 million and give them a substitute player, perhaps Benny Agbayani.
The doctors — who indicated their interest in a conference with a Brooklyn judge on Thursday — would face serious obstacles, not the least of which would be getting commitments from a partner who might or might not see them as an asset in running the hospital, in Cobble Hill.
They indicated their interest at the International Island Games Association's (IIGA) annual meeting in Jersey earlier this month.
Trainers involved in the project indicated their interest in determining the students' understandings of whole number-place value, fractions and money.
Lekwungen participants have also indicated their interest in having more edible native species throughout the islands, particularly species found in small numbers such as trailing blackberry [Rubus ursinus subsp. macropetalus (Douglas ex Hook). Roy L. Taylor & MacBryde], Saskatoon berry [Amelanchier alnifolia (Nutt). Nutt. ex M. Roem.], and the more abundant Pacific crabapple [Malus fusca (Raf).].
And the World Bank, the European Investment Bank and the Waitt Foundation indicated their interest in supporting the replication and scaling of these efforts.
With the possibility of international companies bidding on the project, the Soviets who had just installed a 500 MW hydroelectric generator on the Yenisei River indicated their interest.
"Later today, we'll begin providing information about the event deletion to the approximately 2,600 users who indicated their interest in the event, and the 600 plus users that said they'd attend".
