The NFL announced the dates for various minicamp and offseason training activities across the league. Teams are permitted to hold a rookie minicamp on one of the first two weekends after the NFL Draft and conduct one mandatory minicamp for veteran players.
Teams with new head coaches are permitted to conduct one additional voluntary minicamp for veteran players. There is no restriction on the number of minicamps a club may hold for rookie players. I think the 49ers should petition for an additional veteran minicamp since they did not get such a minicamp last year thanks to the lockout!
The 49ers are currently involved in their voluntary workout programs, with players starting to conduct various drills with their position coaches. The 49ers rookies will have a minicamp from May 11 to May 13. They will then conduct OTAs May 22 to 24, May 29 to 31 and June 4 to June 7. They will wrap things up with a mandatory minicamp June 12 to 14. I don't actually know what the difference is between OTAs and minicamps as the CBA does not appear to detail that.