Some info from the Prospect camps thread on the Prospect board.
For a European player, in the past there have been cases where there were visa issues complicating ability to attend the summer camp, even for a drafted player.
For a European player maintaining NCAA eligibility, same rules as other NCAA players about paying cost of attending camp and that ends up being cost prohibitive in some of those cases. It is also why you will see some drafted current or future NCAA players not appear on the rosters once they are released, because any player maintaining NCAA eligibility has to pay the costs to attend.
One more point, the CBA (8.9) rules are different for signing a European prospect in the off-season if the player is younger and did not play in North America for the entire prior season. As we have seen in the past, with Nathan Walker/WSH and Tomas Hyka/PHI two high profile examples, some players attend but are ineligible to be signed. Does not prevent team from inviting or a player from attending, but it does change the situation for younger European prospects who did not play all of last season in North America.
CBA 15.10 Conditioning Camp.
(a) In each off-season, a Club may hold one conditioning camp(a "Conditioning Camp") for the Players set forth in Section 15.10(b) below, provided that such Conditioning Camp be limited to the months of June and July only and may not last longer than seven (7) days, and provided further that such Club shall advise the NHL and NHLPA of the time, duration, and location of its Conditioning Camp at least two (2) weeks in advance, in accordance with Exhibit 3.
(b) Participation in Conditioning Camps will be limited to the following:
(i) Unsigned Draft Choices;
(ii) Any Player in the Entry Level System with less than 120 NHL Games played (but who did not play in excess of 70 NHL Games in the preceding Regular Season);
(iii) Draft-related Unrestricted Free Agents;
(iv) Players who were eligible for selection in the preceding Entry Draft but were undrafted; and
(v) Any Player who is not in the Entry Level System, who finished the preceding season disabled, and who submits a written request to the Club, with a copy to the NHL and the NHLPA, to participate in his Club's Conditioning Camp.
(c) Ice-time during Conditioning Camp will be limited to no more than three (3) hours per day, per Player.
(d) No Player shall be required to participate in more than three (3) Conditioning Camps in total for any one (1) Club (but may opt to do so in his sole discretion).
(e) A Conditioning Camp may be held in a location of the Club's choice, as long as the arena facility and hotel/lodging accommodations are of suitable standard for professional hockey players. Clubs shall pay all reasonable expenses for the Players participating in a Conditioning Camp, including, without limitation, travel and lodging. Players participating in Conditioning Camp are also entitled to the same per diem meal allowances as provided in Section 19(a) of this Agreement (subject to the appropriate offsets in the event the Club provides breakfast and/or lunch, as per Section 15.2(b)).