Comparable entry, according to Adler, in the Ambrosian Lexicon. Both the headword (evidently quoted from somewhere) and the participle in the gloss are aorist middle participles, nominative singular masculine. See also pi 2692.
For the verb prosetairi/zomai (best known for its occurrence in Herodotus 6.66.2, on Kleisthenes' political positioning in late-sixth-century-BCE Athens: to\n dh=mon prosetairi/zetai) see also under lambda 254.